Simplify and generalize remarkup engine construction

Summary:
Depends on D6395.

  - Now that inline rules have explicit priorities, they can just go in applications in all cases.
  - We don't need the inline rule conditionals anymore after D6395.

Test Plan: Wrote remarkup with mentions, phriction links, countdowns, etc.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6396