R1:7ad4c9c056d1

Replace Harbormaster "BuildItem" with Lint/Unit messages

Summary:
Ref T8095.

Harbormaster has a `BuildItem` class, but it has no table and is unused. This was an earlier idea about representing lint/unit results and some other possible types of messages, but I think we want to be more specific than this.

Remove `BuildItem` and add `Lint` and `Unit` storage. These tables roughly parallel how we store lint/unit messages today, with some guesses about how where they'll go in the future.

Test Plan: Ran…