Extension to Housekeeping for cleaning up stale branches or branches without an open MR

This was a question I was wondering about. I can only imagine all the times you don't push the checkbox "delete this branch" when opening a Merge Request. This maybe adds up in total space across gitlab.com or your gitlab instance. One button to do this.. along with a scheduling function + feedback to individual user that this is going to happen in a few days time, is perhaps something that we want?

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Basically a button to clean up all branches throughout the project or even instance without an open MR or stale branches in one fell swoop?

cc: @markpundsack @pcarranza @JobV