Prevent a 500 when a repository's root_ref is nil

Closes #14259

See merge request !3278

Dashboard labels

Previously because there were no JSON endpoint for labels or milestones
it was fetching HTML and parsing that. This is wrong.

It now fetches from a JSON endpoint.

This also fixes the dashboard/issues page not loading labels or
milestones as the path used to be only for a single project. So instead
I have created a endpoint for labels & milestones JSON on dashboard.

See merge request !3253

Fixed other issues based on feedback

They would try to load JSON from a project even though it isn't a single project

Fixes issue with close button not working on MR

Closes #14383

See merge request !3295

Improved issue sidebar

Updated the spacing throughout the sidebar so that it fits better on smaller screens. Also adds a more participants button to show any other participants after the first row.

Closes #13353

See merge request !3097

Add note in upgrade guide to choose the correct version tag

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3448

@rspeicher I'm assigning to you since as a release manager you should know better. Let's keep it in this guide for now and then start copying it in the future ones.

See merge request !2284

[ci skip]

Fix MergeRequest#source_sha when there is no diff

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14170

### Overview

This MR fixes an unhandled Exception when visiting the page of an open Merge Request  without diff.

### Description

`MergeRequest#source_sha` is expected to return the sha of the source branch last commit. But when an open Merge Request has no diff (e.g. all commits have already been merged to the target branch), `merge_request.source_sha` incorrectly returns `nil`.

This was without consequences before – but since !2217 was merged (a few days ago), it makes `Gitlab::Git::Commit.between` raise an "Unexpected nil argument" exception. This can be reproduced when visiting the http://localhost:3000/gitlab-org/gitlab-test/merge_requests/2 page on a fresh local Gitlab setup.

This MR fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available. I also added tests.

@DouweM I believe you wrote most of this code in the first place ; does this looks correct to you, or is there a better way to resolve this issue maybe?

See merge request !3135

Changing filter dropdowns shows loading

Instead of doing a full refresh of the page - i've modified the filterResults method on the Issues object to work for this form as well

![issues](/uploads/3335f09f7df88b6d419de7fd3d6857d2/issues.gif)

Closes #14359

See merge request !3237

Add confidential issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3678

More information: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/227

See merge request !3282

Show a notice for diffs that are empty/too large

This builds upon https://gitlab.com/gitlab-org/gitlab_git/merge_requests/72 but will fail until said MR has been merged.

Fixes gitlab-org/gitlab-ce#13912, gitlab-org/gitlab-ce#13644, gitlab-org/gitlab-ce#13557, gitlab-org/gitlab-ce#13267, gitlab-org/gitlab-ce#3906

See merge request !3185

[ci skip]

Fixed bug where wouldn't work on MR index

Instead of doing a full refresh of the page - i've modified the filterResults method on the Issues object to work for this form as well

Closes #14383

This builds on the changes introduced in
https://gitlab.com/gitlab-org/gitlab_git/merge_requests/72 and results
in merge requests with large diffs (e.g. due to them containing minified
CSS) loading much faster.

Add space to separate different files in diff

I propose to add some visual separation between files in diff. Originally this separation was removed in 8.0 because of different UI layout. Now when we have white layout back I think it will be better to add some space between files. 

- - - 
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>


See merge request !3275

Removed diverging commit count calculation from Repository#build_cache

Using a repository with 1000 branches the old `Repository#build_cache` method would take around 180 seconds to complete. Without calculating the diverging commit counts this method "only" takes around 60 seconds. 

See commit 28cc2413eb5ddf920ce0b5eed803121f8b884754 for more details. This fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14058

cc @rspeicher 

See merge request !3274

Adds small AJAX optimistic functionality to todos.

Fixes #13656 <br/>
A good first step and boring solution. <br/>
Will make ajax call to remove each issue.  <br/>
If issue is last in group of issues will refresh page.  <br/>
If issues remain in group will remove row with JS.  <br/>
Adds loading spinner to button and disables. <br/>

![todos-optimistic](/uploads/c6aec4a688e5125d4df55a2a3a55d4d7/todos-optimistic.gif) <br/>

cc @dzaporozhets @JobV @dbalexandre 

See merge request !2946

Fixes issue with any milestone value in dropdown not being selectable

Closes #14293

See merge request !3234

Closes #14293

Set global JS variable to sync view up

Fixed button hover color

Updated the spacing throughout the sidebar so that it fits better on smaller screens
Added more participants button

Closes #13353

Add side padding to content-list if inside panel
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fixes #14367, #14349

See merge request !3273

Use relative links in `doc`, fix broken generated HTML links

Links like (doc/web_hooks/web_hooks.md) work in the GitLab source code
web interface, but the HTML generator produces broken links in the `doc`
subdirectories:  
http://doc.gitlab.com/ce/hooks/doc/web_hooks/web_hooks.html  
instead of the right one  
http://doc.gitlab.com/ce/web_hooks/web_hooks.html  
in  
http://doc.gitlab.com/ce/hooks/custom_hooks.html.

Fixes #14338 


See merge request !3261

Remove CI duplication.



See merge request !3290