R1:412fc3455730

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…