Render remarkup in feed in a mostly reasonable way

Summary:
Fixes T4057. This sort of sidesteps the trickiest (but very rare) case of things like embedded slowvotes. We might be able to refine that later.

In the common bad case (macros, large images) it gets reasonable results by using overflow: hidden with max-height.

We use PhabriatorMarkupEngine::summarize() to try to just render the first paragraph.

Test Plan:

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4057

Differential Revision: https://secure.phabricator.com/D10355