Move notifications and todos to header buttons (merge requests)

What does this MR do and why?

• Moves notifications and todos to buttons in page header for the merge request page (issues/epics/incidents will be updated in separate MRs)
• See https://gitlab.com/gitlab-org/gitlab/-/issues/426142+ for more details
• Rollout issue: [Feature flag] Enable :notifications_todos_buttons (#426305)
• :moved_mr_sidebar will be removed in Removes moved_mr_sidebar flag (!127001 - merged)

Screenshots or screen recordings

How to set up and validate locally

• Enable :notifications_todos_buttons
• Look at merge request
  • Add a todo, subscribe/unsubscribe, scroll down etc
• Should not affect any other issuables

UX Paper Cuts 16.5 → Cleanup (#424403 - closed)