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 bin/storage upgrade and got a clean adjust out of it.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: hach-que, epriestley

Maniphest Tasks: T8095

Differential Revision: https://secure.phabricator.com/D13329