Raise a warning when mentioning a user in a comment on a draft revision

Summary: See PHI433. Ref T13102. Users in the wild have mixed expecations about exactly what "draft" means. Recent changes have tried to make behavior more clear. As part of clarifying messaging, make it explicit that @mention does not work on drafts by showing users a warning when they try to @mention a user.

Test Plan:

• Mentioned users on drafts, got a warning.
• Posted normal comments on drafts, no warning.
• Posted normal/mention comments on non-drafts, no warning.

Maniphest Tasks: T13102

Differential Revision: https://secure.phabricator.com/D19210