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