Disable commenting on unrelatable diff line

## What does this MR do?

Fixes a bug where users can comment on diff lines that can't be commented on and attached to.

This is the case for unfolded lines and lines that appear as snippets in the discussion tab.

**!5864 should be merged before this MR.**

## Are there points in the code the reviewer needs to double check?

😕 

## Why was this MR needed?

Comments were getting lost on the discussion feed, unable to find their related diff.

## What are the relevant issue numbers?

Closes #20633.

## Screenshots (if relevant)

![diffs-comment-fix](/uploads/f902b687fda75492e38397f9705283e0/diffs-comment-fix.mp4)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5681

Fix file links on project page Files view

## What does this MR do?

Fix the project show page when the default view is set to Files so that the links work.

## Are there points in the code the reviewer needs to double check?

I can't figure out what caused this to break 😕 

## What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-com/support-forum/issues/959.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md) (N/A)
- [x] API support added (N/A)
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5933

Fix Spinach branches spec

I broke master by adding a branch to gitlab-test for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5961, which put the branch in this scenario off the first page 😆 

See merge request !5964

Also check if Akismet is enabled, before showing the `Submit as spam` button.

Fixes #21229

See merge request !5948

Fix incorrect "stopped impersonation" log message

Minor bug: Usernames were swapped in the log message.

Closes #21015

See merge request !5949

Fix project namespace links

#### What does this MR do?
Fixes pipeline links

#### Why was this MR needed?
Links went to a 404 on fork MR pipelines tab

#### What are the relevant issue numbers?
Closes #21107

See merge request !5912

entities: make Environment inherit EnvironmentBasic

## What does this MR do?

Makes the `Environment` entity inherit from `EnvironmentBasic`

## Are there points in the code the reviewer needs to double check?

No.

## Why was this MR needed?

Just a code simplification.

## What are the relevant issue numbers?

N/A

## Screenshots (if relevant)

N/A

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5959

Change merge_error column from string to text type

See merge request !5950

Fix typo in gitlab_flow.md

## What does this MR do?

Cross MR from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/674

See merge request !5960

In some cases, the artificial 255-character limit would cause an error in
MergeWorker.

Closes #20593

Fixed enter key in search input not working

## What does this MR do?

Fixes a bug where the enter key wouldn't search in the top nav search input

Closes #20627 

See merge request !5888

Closes #20627

Closes #21015

Remove tab stop from issuable form added by description templates

See merge request !5929

Revert the revert of Optimistic Locking

## What does this MR do?

It reverts the revert of optimistic lock https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5146 (and revert MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5245)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19871

Actually the code is already reviewed, except:
* Monkey patch `config/initializers/ar_monkey_patch.rb`
* We removed default values from migration `db/migrate/20160707104333_add_lock_to_issuables.rb`

See merge request !5623

Fix location of markdown help

Resolves #21208, and incorrect link to the markdown style guide

See merge request !5945

Tests for diff notes (without failures 🙏)

## What does this MR do?

Adds specs for diff notes.

## Are there points in the code the reviewer needs to double check?

We need to ensure that this doesn't break master again.

## Why was this MR needed?

## What are the relevant issue numbers?
Closes #20957
## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5864

Fix for update 8.10-to-8.11.md doc.

## What does this MR do?

Fixed a typo in the docs to update Ruby.

See merge request !5941

Bump SimpleCov merge timeout to 365 days

## What does this MR do?

In case that CI pipeline fails because of intermittent errors in one of the jobs, when someone retries the only failing job located in the middle stage, the job that calculates compound coverage that is configured in later stage, and triggered after retrying job in the pipeline, will still return an accurate coverage value.

Closes #21144

See merge request !5932

Add Ruby 2.3 upgrade notes.

The 8.10 to 8.11 docs didn't mention Ruby 2.3, now they do.

See merge request !5940

Split Koding admin/user docs

Backend/Frontend MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5909

See merge request !5938

Documentation for Issue Boards

## What does this MR do?

Add documentation for issue boards.

## What are the relevant issue numbers?

- Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/17907
- Backend MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5548
- Frontend MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5554

See merge request !5713