Improve inline mail snippet rendering, possibly fixing Airmail?

Summary:
Ref T10694. General improvements:

• Remove leading empty lines from context snippets.
• Remove trailing empty lines from context snippets.
• If we removed everything, render a note.
• Try using style instead of <pre>? My thinking is that maybe Airmail has weird default rules for <pre>, since that's the biggest / most obvious thing that's different about this element to me.

Test Plan: Viewed normal comments locally, faked a comment on an empty line in the middle of a lot of other empty lines.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10694

Differential Revision: https://secure.phabricator.com/D15864