Add ability to edit build messages after the fact

Just banged this out real quick as a PID.  It's [available on staging](https://staging.commcarehq.org/a/ethan-test-domain/apps/view/f82326c7b41a9922036d79e17439bf9f/releases/?edit=true) to play with.

@biyeun I kinda just winged it, can you take a look and tell me what buttons I should really be using?  It's a very tiny interface, so I'm not sure if big `btn-primary`s are the answer, but happy to defer judgement.
@amsagoff FYI