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