Added review mode to new IDE sidebar

What does this MR do?

Adds review mode link to the IDE left sidebar. It also removes the dropdown to switch between edit & review mode.

This functionality is the same as opening files from the commit section. However, it is now possible to edit in 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)