feat: LEAP-1474: Create and display comments with classifications

[jombooth]

Implements classifications on comments, leveraging new-style taxonomy. Classifications are loaded from and persisted to the DB. Also allows making a comment that's classifications-only and has no text.

Classifications are non-editable for this PR.

A couple examples of comments with associated regions, one with text and one without:

Comments without associated regions with and without text:

Comment interface when org has a classification config, with and without linking mode:

Comment interface when org has no classification config, with and without linking mode:

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