Basic Setup for MR Showing

What does this MR do?

⚠ EE Port gitlab-org/gitlab-ee!5147

• Show button in MR
• Open all files from MR (also deep files)
• Load correct base version and display correct diff
• Put current mode in state
• Show mode Dropdown to change between edit + reviewing changes
• Color DD button based on mode
• Files get special Icon when part of MR? (git-merge atm)

Why was this MR needed?

To make the Web IDE capable of opening MR.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#44840, gitlab-org/gitlab-ce#44839