Implement cleanup actions for branches when the PR has already been merged

[justinrobots]

When a local branch is associated with a PR that has already been merged, provide paths for users to cleanup the branch instead of showing a merge indicator that prompts them to rebase/merge, or a button that prompts them to "publish branch".

From @eamodio:

this gets into the "changes" we've started to talk about for something that is "stale" (and that doesn't just mean old). Definitely want a good "clean up" workflow and adaption of the UI