In HTML mail, make the text for mail stamps in mail bodies smaller and lighter

Summary: Depends on D19014. Ref T13053.

Test Plan: Used ./bin/mail show-outbound --id <id> --dump-html > out.html && open out.html to look at HTML mail, saw smaller, lighter stamp text with better spacing.

Reviewers: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D19015