Don't show meme Remarkup hint button if Macro application is not usable

Summary: See <https://phabricator.wikimedia.org/T906>. This behavior is a bug; we should remove the button if the user can't use the application.

Test Plan:
- With Macro uninstalled, did these things verifying the button vanished:
  - Sent a user a message.
  - Edited a revision.
  - Edited repository basic information.
  - Edited an initiative.
  - Edited a Harbormaster build step.
  - Added task comments.
  - Edited profile blurb.
  - Edited blog description.
  - Commented on Pholio mock.
  - Uploaded Pholio image.
  - Edited Phortune merchant.
  - Edited Phriction document.
  - Edited Ponder answer.
  - Edited Ponder question.
  - Edited Slowvote poll.
  - Edited a comment.
- Reinstalled Macro and saw button come back.
- Used button to put silly text on a funny picture.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10900