Pre-prepare for hovercards

Summary:
D5120 and followups refactor and generalize object references in Remarkup -- notably, they move remarkup rules from a central location to the implementing applications.

Preserve blame by doing moves/renames only first. This change moves application remarkup rules into those applications, and renames the ones D5120 modifies.

Test Plan: Typed some preview text into a textarea, got a valid Remarkup render.

Reviewers: vrana, chad

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5123