Modularize almost all Harbormaster build message workflows and UI/UX
Summary: Ref T13072. Push nearly all Harbormaster build message logic into the new per-message transaction classes.
Test Plan:
- Issued every message to Buildables.
- Issued every message to Builds.
- Looked at a big pile of error messages, couldn't find any typos.
- Grepped for affected symbols, etc.
- Ran bin/harbormaster restart ....
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13072
Differential Revision: https://secure.phabricator.com/D21691