Clean up Ponder CSS

Summary: Ref T3578, Removes old cruft and updates Ponder UI very slightly.

Test Plan: Use Ponder alot, new answers, comments, questions. Grep for removed CSS.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T3578

Differential Revision: https://secure.phabricator.com/D13786