Fix when open from MR

• Receives mrId and mrTargetProject. This is the way the old Web IDE worked
• Adds displaying the source branch in the status bar
• Does not show "Create MR" if already opened from an MR

Screenshots

Opening an MR from the example app is a bit weird for now...

• Leave Git Ref empty.
• Add the MR ID to the URL param (for example http://localhost:8000?mrId=7)
• The Project Path should be the path the branch is on (this is how the current Web IDE works).
  • If the source / target project of the MR are the same, this is easy.
  • If the source / target project of the MR are different, the Project Path should be the source project and add a url parameter for mrTargetProject (for example http://localhost:8000?mrId=7&mrTargetProject=gitlab-org%2Fgitlab-ui

With MR from same repo	With MR from fork
20221215_open_mr_same	20221215_open_mr_fork