Make Herald adapters provide content types

Summary:
Ref T2769. Get content types out of hard-coded config and into dynamic adapters.

This removes the "MERGE" and "OWNERS" content types, which were vestigal. These needs are likely better addressed through subscriptions/transactions, and are obsolete, and haven't existed for 2+ years and no one has asked for them to be restored.

Test Plan: Mostly a bunch of grep. Viewed rule list, rule edit. Edited a revision.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2769

Differential Revision: https://secure.phabricator.com/D6656