Partially generalize Remarkup previews and add support to Differential

Summary:
Ref T3671. A lot of applications have pretty ad-hoc preview code. Clean it up a bit and add Summary preview to Differential.

After ApplicationTransactions we might want to try to serialize the whole form and show a preview of all the transactions, but this seems not very useful in most cases (I'd guess that Remarkup previews are 99% of the value) and tricky to get right (e.g., adding images which don't exist yet to Pholio mocks).

I think I can add this in a few other places, too.

Test Plan:
Edited Maniphest Tasks and Differential Revisions, mashed some buttons. Verified previews rendered correctly. Grepped for removed CSS classes (no hits).

{F52907}

Reviewers: btrahan, Firehed

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T3671

Differential Revision: https://secure.phabricator.com/D6673