Make Differential inline comment rendering more consistent and somewhat more modern
Summary: Ref T2222. Ref T1790. I partially modernized this recently, but bring it to the mail version too.
Test Plan: See screenshots.
Reviewers: btrahan
Reviewed By: btrahan
CC: zeeg, aran
Maniphest Tasks: T1790, T2222
Differential Revision: https://secure.phabricator.com/D8294