Prepare for hovercards
Summary:
- Unify all the reference/embed Remarkup rules for Differential, Maniphest, Paste and Ponder.
- Add rules for Pholio.
- Does not yet unify Diffusion or Files (both are a bit more involved).
- Prepare for hovercards.
Test Plan: {F33894}
Reviewers: chad, vrana
Reviewed By: vrana
CC: aran
Differential Revision: https://secure.phabricator.com/D5120