Modularize individual Harbormaster build messages

Summary:
Ref T13072. Further modularize build messages by applying each one in a separate transaction type.

This makes it easier to add new types of messages (although I have no particular plans to do this, offhand) and reduces the amount of switch-boilerplate.

This will probably also simplify validating "harbormaster.sendmessage".

Test Plan:

• Applied all commands.
• Ran migration, saw transactions render properly

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13072

Differential Revision: https://secure.phabricator.com/D21690