Generalize the markup engine factory

Summary:
This thing services every app but it lives inside Differential right now. Pull
it out, and separate the factory interfaces per-application.

This will let us accommodate changes we need to make for Phriction to support
wiki linking.

Test Plan: Tested remarkup in differential, diffusion, maniphest, people,
slowvote.
Reviewed By: hsb
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, hsb
Differential Revision: 646