Check if selected object is valid before passing to calback

See: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/974#note_20210158

See merge request !977

Always use `fixture_file_upload` helper to upload files in tests.

See merge request !975

* Also is not a good idea to use File.open without closing the file
  handler. We should use it with a block or close it explicitly.

Port of "Add Mattermost Service" to EE

See merge request !972

Update the "remove an estimation" command.

The current documentation is not up to date with the current Gitlab version. 

The current command is `/remove_estimate` to remove estimation time of an issue.

See merge request !970

Enable Style/MultilineOperationIndentation in Rubocop

Fixes #25741

See merge request !8125

Mattermost session

This branch is based of tag v8.14.4 so I could test on an instance better.

But stuck on the tests, as setting up the whole doorkeeper stuff doesn't seem to be supported for testing.

See merge request !8033

Don't delete branches/tags that contains changes only in the remote mirror.

Closes #588

See merge request !968

Fix restart link on doc/pages/administration.md

The links for `restart_gitlab.md` and `backup_restore.md` both had an extra `../`, resulting in a `404` on https://docs.gitlab.com/ee/pages/administration.html#nginx-configuration

As mentioned in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/967#note_20134955

See merge request !973

Cleaner merge commit messages

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/23462

See merge request !7722

Add sorting functionality for group/project members

## What does this MR do?

This MR adds sort functionality to group/project members list.

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

No.

## Why was this MR needed?

This MR helps the user to manage and find users of a group/project fast. It also improves UX.

## Screenshots (if relevant)

**Before**
![before-pr-member](/uploads/412bfdfddc63e48a62e2095d664660b5/before-pr-member.png)

**After**
![pr-after](/uploads/ae23a5c7689b46d26a948f22a0ac2360/pr-after.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #21622 


Closes #23573

See merge request !7032

Avoid use of Hash#dig to keep compatibility with Ruby 2.1

See merge request !8153

Refactor Bitbucket importer to use BitBucket API Version 2

## What does this MR do?

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

## Why was this MR needed?

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/19946

## Screenshots (if relevant)

This MR needs the following permissions in the Bitbucket OAuth settings:

![image](/uploads/a26ae5e430a724bf581a92da7028ce3c/image.png)

- [] 

## 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 !5995