Summary:
Provide a viewer to all remarkup engines.
This fixes commit summaries in Diffusion, which were failing to link because they didn't have a user and thus couldn't see/load `D123`, e.g.
Test Plan: Grepped for engine creation.
Reviewers: vrana
Reviewed By: vrana
CC: aran, edward
Maniphest Tasks: T603
Differential Revision: https://secure.phabricator.com/D5152
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
