Modernize "macro" typeahead datasource

Summary: Ref T4420. Make this modern.

Test Plan:

• Used typeahead in remarkup comment area to select macro "derpdog".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9875