R1:8bbee921398d

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

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