Uncheat Macro datasource browse support

Summary: Ref T5750. Macro browse support is actually useful (in the meme dialog) and fairly straightforward to implement.

Test Plan:

• Browsed macros.
• Queired macros.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5750

Differential Revision: https://secure.phabricator.com/D12439