Implement "words of power" against ApplicationTransactions

Summary:
Ref T2222. Differential has certain "words of power" (like `Ref T123` or `Depends on D345`) which should expand into a separate transaction when they appear anywhere in text.

Currently, they're respected in only some fields. I'm expanding them to work in any remarkup field, including comments and inline comments.

This partially generalizes transaction expansion/extraction in comments. Eventually, I'll probably implement some very soft sort of reference edge for T4036, maybe.

Test Plan: {F119368}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8369