WIP: Resolve "Merge request diffs in multi-file editor"

What does this MR do?

• Show button in MR
• Open all files from MR (also deep files)
• Put current mode in state
• Show mode Dropdown to change between edit + reviewing changes
• Show Tabs
• Color DD button based on mode
• Files get special Icon when part of MR?

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #4581 (moved)