Add a download buttons for Build Artifacts

## What does this MR do?
This MR adds a download buttons for build artifacts of latest succesful pipeline in:
- dashboard of project,
- branches and tags views,
- and tree viewer

Implement #4255

## What are the relevant issue numbers?

Closes #4255, Closes #14419

## Screenshots

### Project main

![](/uploads/29ee2154a214416059a875f2715d4fa3/Screen_Shot_2016-08-24_at_8.00.31_PM.png)

### Branches

![](/uploads/9220c593288370986fbc1d42a1425ef7/Screen_Shot_2016-08-24_at_8.02.01_PM.png)

### Tags

![](/uploads/a843e8103221fea475a0cf9d62a1999d/Screen_Shot_2016-08-24_at_8.03.32_PM.png)

### Source Tree

![](/uploads/63cd3c8c91b6f427c166dc90d8e3c059/Screen_Shot_2016-08-24_at_8.04.56_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [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)
- [x] Download buttons
- [x] Models
- [x] Routes
- [x] Projects::ArtifactsController
- [x] API
- Tests
  - Rails
      - [x] Project#builds_for
      - [x] branch name with slashes
      - [x] only success builds
      - [x] only latest builds
      - [x] feature tests for download buttons
  - API
      - [x] branch name with slashes
      - [x] only success builds
      - [x] only latest builds
  - [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 !5142

Add test for closed MR without fork

Add view test visibility of Reopen and Close buttons

Fix controller tests and validation method

Fix missing space

Remove unused variables from test

closed_without_fork? method refactoring

Add information about missing fork

When closed MR without fork can't edit target branch

Tests for closed MR edit view

Fix indentation and rebase, refactoring

Add horizontal scrolling to all sub-navs on mobile viewports

## What does this MR do?
Adds horizontal scrolling to all the sub navigation on mobile views. This was previously only available on the repository tab (commits page). 

## Are there points in the code the reviewer needs to double check?
Perhaps a better way to refactor out the common haml code

## Why was this MR needed?
Adding it to all to keep the UI consistency and to offer mobile users a better interface

## What are the relevant issue numbers?
Closes #20779 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-23_at_3.52.35_PM](/uploads/3833da4252cf616266a77654cee89faf/Screen_Shot_2016-08-23_at_3.52.35_PM.png)

After:
![Screen_Shot_2016-08-23_at_3.52.21_PM](/uploads/5a023bb1e639f16f2471009dd6ae0d62/Screen_Shot_2016-08-23_at_3.52.21_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry 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 !5762

* upstream/master: (107 commits)
  Fix external issue tracker "Issues" link leading to 404s
  Fix CHANGELOG entries related to 8.11 release.
  Fix changelog
  Reduce contributions calendar data payload
  Add lock_version to merge_requests table
  Add hover color to emoji icon
  Fix wrong Koding link
  Capitalize mentioned issue timeline notes
  Fix groups sort dropdown alignment
  Use icon helper
  Fix inline emoji text alignment
  Adds response mime type to transaction metric action when it's not HTML
  Moved two 8.11 changelog entries to 8.12
  Fix markdown link in doc_styleguide.md
  Display project icon from default branch
  Reduce number of database queries on builds tab
  Update CHANGELOG
  Update Issue board documentation
  Label list shows all issues (opened or closed) with that label
  Update CHANGELOG
  ...

Add hover color to emoji icon

## What does this MR do?
Adds hover color to emoji icon in discussions

## Are there points in the code the reviewer needs to double check?
Shouldn't be

## Why was this MR needed?
Keep consistency of color highlighting to indicate action icon

## What are the relevant issue numbers?
Closes #20629 

## Screenshots (if relevant)
Before:
![Gcy0V4Cjp9](/uploads/96a7a683fe2d526b74a05d7cc91c9fa0/Gcy0V4Cjp9.gif)
After:
![TAmusA6o1z](/uploads/1baf20390d2f1442b5fae84ae42914ce/TAmusA6o1z.gif)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry 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 !5673