R1:f5ff10fe283d

Put inline previews in remarkup textareas

Summary:
Ref T3967. This gives us a reasonable baseline for doing remarkup previews inline in all contexts, and works in weird/constrained context including:

- inline comments;
- conpherence; and
- custom fields.

It would be nicer to go beyond this in contexts like Phame posts, but this is a start, at least.

Test Plan:
{F1040877}

{F1040878}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3967

Differential Revision: https://secure.phabricator.