Add inline styles for {key ...} in HTML mail

Summary: Fixes T11607.

Test Plan:

• Made a comment using {key ...}.
• Used bin/mail show-outbound --id X --dump-html > test.html to review HTML:

{F1805304}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11607

Differential Revision: https://secure.phabricator.com/D16523