Render prose diffs in email as summaries

Summary:
Ref T7643. When a large block of prose text is edited (like a wiki page), summarize the diff when sending mail.

For now, I'm still showing the whole thing in the web UI, since it's a bit more manageable there.

Also try to fix newlines in Airmail.

Test Plan:
This web diff:

{F1682591}

..became this mail diff:

{F1682592}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16098