Add cached summarization to PhamePost

Summary: Restore summarization. Use the remarkup cache, and try to do it somewhat-intelligently (pick the first paragraph that looks like it's text).

Test Plan:
{F21323}

{F21324}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1373

Differential Revision: https://secure.phabricator.com/D3715