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 (closed)

What are the relevant issue numbers?

Closes #4255 (closed), Closes #14419 (closed)

Screenshots

Project main

Branches

Tags

Source Tree

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Download buttons
• Models
• Routes
• Projects::ArtifactsController
• API
• Tests
  • Rails
    • Project#builds_for
    • branch name with slashes
    • only success builds
    • only latest builds
    • feature tests for download buttons
  • API
    • branch name with slashes
    • only success builds
    • only latest builds
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together