Web IDE exit button isn't obvious

With the introduction of the new merge request switcher, it's hard to work out how to get back to a project. We need to rethink the way a user can exit the Web IDE.

Original Proposal

### Proposal

• Replace the back button with the project avatar, and just make it go to the project homepage to match standard navigation patterns
• Replace merge request and branch name with individual drop downs with clearly designated click zones
• Opening a branch that is part of an open merge request should automatically detect this and make merge request review modes available and link to the merge request. It is confusing that opening the branch in two different ways results in different capabilities being available. Furthermore, if I need to reload the page for some reason, all the MR features disappear because we end up back in the 'dumb' branch mode.

Design

• Replace the exit button with the project avatar, and just make it go to the project homepage to match standard navigation patterns. Switching MR or branches shouldn't affect this.
• Replace merge request and branch name with the project's namespace
• The MR and branch switcher design is handled in https://gitlab.com/gitlab-org/gitlab-ce/issues/46165