R1:10b86c2aa34d

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.