Modularize individual Harbormaster build messages
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".
- Applied all commands.
- Ran migration, saw transactions render properly
Maniphest Tasks: T13072
Differential Revision: https://secure.phabricator.com/D21690