Merge master to branch in the UI

A lot of non technical team members have a hard time merging master to their branch when working on MRs.

The velocity of www-gitlab-com makes it easy for branches to be quickly out of date (500+ commit differences) within a week of opening a MR. Sometimes, the branch created on the web ide was also from a commit with a failing pipeline (the easiest way to fix their branch is to merge master to their branch once master is green again).

In a perfect world, it would be really cool if MRs had a setting that would allow gitlab to automatically merge master to my branch (to keep it from deviating too much) as long as it doesn't create merge conflicts.

For a first iteration, we should just add a button to merge master to branch