R1:79fa8e40cb11

Implement #yoloswag

Summary:
Ref T3190. #shipit

Likely future work:

- Extract project mentions from remarkup for ApplicationTransactions; this isn't relevant in any apps right now (but will be in Pholio before tooo long). Ref T3189.
- Allow projects to have alternate short names. As written, this is fine for most projects ("Differential" is `#differential`) but not so great for other projects ("Phabricator Public & Media Relations" is `#phabricator_public_media_relations`). This also breaks refs…