R1:1212dc5fbe8b

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…