Improve HTML mail rendering of inline patches

Summary: Fixes T9790. This uses a simple renderer, like the inline context renderer, that emphasizes getting a quick glance at small changes and working reasonably on mobile devices.

Test Plan:

• Set inline setting to 9999.
• Created a diff.
• Saw it render reasonably in HTML mail.
• Also tested text mail to make sure I didn't break that.

{F1310137, size=full}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9790

Differential Revision: https://secure.phabricator.com/D15901