Allow revisions to be commandeered

Summary:

• Adds "Commandeer Revision", to allow you to plunder revisions from those lost to sea (e.g., interns who have left or co-workers who are dealing with a family emergency).
• Removes admin-abandon to simplify things, since you can just Commandeer + Abandon now.
• There are other workarounds available but this is the natural/expected workflow (and the one everyone always asks for) and there's no real reason not to allow it.

Test Plan: Swashbuckled.

Reviewers: cpiro, btrahan, vrana, jungejason

Reviewed By: cpiro

CC: aran, zeeg

Differential Revision: https://secure.phabricator.com/D2257