"Update Branch" button for stale branches #1236

Similar to what was discussed by @vanniktech in #1105 but not exactly the same, because I have no interest in a "hot key" - just a button.

Summary:

If you enable branch protection for a repository and select "Require branches to be up-to-date before merging", then you get an Update Branch button whenever the branch goes stale.

However it would be convenient to have this for all my PRs, including ones in repositories that don't enforce this branch protection.

As described by @hkdobrev in #1105:

This button would merge the default branch in the PR branch with a merge commit if there are no conflicts.

It might be a little tricky to work out who has write access to that branch when it's coming from a fork. In theory this feature/button could initially work for branches within the same repository only though.