R1:68d90ef6986d

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.