Basic @mentions support

Summary:
Provides basic Remarkup support for @mentions. No application integration yet so
these aren't terribly useful until that happens.

Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-83d68e7af6085ae928df/

Reviewers: tomo, mroch, jsp
Commenters: tomo
CC: aran, tomo, epriestley
Differential Revision: 517