at-mentions

— DT + RW

Comes with a migration to escape existing `comment.body`.

https://www.pivotaltracker.com/story/show/82072770

TODOs
- [ ] get reviews from people
- [ ] fixup into one commit, include tracker# in commit message

You are right! we have changed it to use this strategy. Yes, a doc PR is definitely in order. Would you like the open source glory? ;) — DT