Added merge request review to IDE sidebar

What does this MR do?

Adds merge request mode to the IDE sidebar. When the wed IDE is opened from a merge request we automatically switch to the review mode & show to the user they are reviewing a merge request.

There is also a dropdown to allow the user to switch merge request & normal review mode.

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
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#44846 (closed)