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