Make Differential comments be styled similarly to Maniphest comments

Summary:
This diff restructures the DOM and alters some CSS within differential.
Original goal was to unify these codepaths more fully into a base class or
classes, but they have quite a bit of custom code such that didn't feel too
compelling in practice. It also felt related to feed stories as I thought
about the more general version(s) of this code...

Also deleted some CSS from maniphest that wasn't doing anything.

Test Plan:
looked at a differential diff and liked what I saw. spent a bunch
of time trying out different types of comments and etc.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T803

Differential Revision: https://secure.phabricator.com/D1513