Add revision vote option

[velovix]

When a BHAP is marked as "Ready for Discussion", voters should have an option to revisit the draft stage.

[velovix]

@apockill I'm worried about the ramifications of this. For any change to a BHAP, everybody would have to be rounded up to vote for a revision, which might lead to some user fatigue. Right now it's possible to edit BHAPs even if they're in the "Ready for Discussion" phase, so informal revision requests can be satisfied by the original author directly. This is definitely more informal, but that might be a good thing.