Implement Build Plan "Hold Drafts" behavior

Summary: Ref T13258. Makes the new "Hold Drafts" behavior actually work.

Test Plan:

• Created a build plan which does "Make HTTP Request" somewhere random and then waits for a message.
• Created a Herald rule which "Always" runs this plan.
• Created revisions, loaded them, then sent their build targets a "fail" message a short time later.
  • With "Always": Current behavior. Revision was held as a draft while building, and returned to me for changes when the build failed.
  • With "If Building": Revision was held as a draft while building, but promoted once the build failed.
  • With "Never": Revision promoted immediately, ignoring the build completely.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13258

Differential Revision: https://secure.phabricator.com/D20232