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