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

Shorten task status phrase

## What does this MR do?
Shortens the task status phrase to make it easier to read

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

## Why was this MR needed?
Improve readability/scan-ability of the issues table

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-27_at_12.38.17_PM](/uploads/12d54e4ce24dea203e8f7189b32e3a43/Screen_Shot_2016-08-27_at_12.38.17_PM.png)

After:
![Screen_Shot_2016-08-27_at_12.38.05_PM](/uploads/7dfcc3284025b889f6afa09ca273d928/Screen_Shot_2016-08-27_at_12.38.05_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)

## What are the relevant issue numbers?
Closes #21543

See merge request !6062

User can edit closed MR with deleted fork

## What does this MR do?
User can edit closed MR with deleted fork (can't change "Target branch"). When fork is deleted "Reopen" button is hidden.

## What are the relevant issue numbers?
Closes #19315 

## Screenshots (if relevant)
* hidden Reopen button and information about deleted fork
![Zrzut_ekranu_2016-08-12_o_13.19.24](/uploads/d288c5625e788382e31b2979acb601df/Zrzut_ekranu_2016-08-12_o_13.19.24.png)

* editable fields for closed MR without fork
![Zrzut_ekranu_2016-08-05_o_12.24.38](/uploads/1549e54d4bc2a9939ef296ce66139706/Zrzut_ekranu_2016-08-05_o_12.24.38.png)


## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [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)

cc @ubudzisz @yorickpeterse @grzesiek @tmaczukin

See merge request !5496

Fix diff comments on legacy MRs

## What does this MR do?

Allow diff commenting on MRs without complete diff refs. (Commenting would work before, but the JSON response would fail, so you'd only see your comment after a refresh.)

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

Is this really all I needed to do? I feel like there must be more to it :blush:

## Why was this MR needed?

It's a regression!

## What are the relevant issue numbers?

Closes #21211.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [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 !6029

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