Consolidate merge request main actions in one place
### Blocks
https://gitlab.com/gitlab-org/gitlab/-/issues/233285
### Problem
Currently the main (frequent/important) actions of a merge request are scattered throughout the UI. This causes many problems:
- Decreases feature discoverability
- Increases time spent finding these frequent or important actions
- Increases user's cognitive load
- Relies more on user's memory than intuitiveness
As an example, notice the current placement of the actions highlighted in red:

### Proposal
We should try to consolidate all of the main (frequent/important) actions of a merge request in one single location of the UI, always accessible to the user.
- [WIP in Figma 🔗](https://www.figma.com/file/q2fr4kFx0EFcbUYZhdQIUq/Consolidate-merge-request-main-actions-in-one-place?node-id=1%3A62)
#### Relevant research findings in Dovetail 🕊
- [Overview tab - needs to be improved](https://dovetailapp.com/projects/8caf91dc-8b99-4e05-b040-57428bf55b5c/insights/present/30756ad7-3156-4657-94ed-66576cfeacdc)
- [MR commands center](https://dovetailapp.com/projects/8caf91dc-8b99-4e05-b040-57428bf55b5c/insights/present/b504f952-6ae7-4cca-a028-3417f6a87826)
- [Place 'Merge' option on code review page](https://dovetailapp.com/projects/8caf91dc-8b99-4e05-b040-57428bf55b5c/insights/present/2f3ace3b-3b2d-4da3-ada5-77a00eedf5be)
epic