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