Polish up timeline for PHIUTwoColumnView

Summary: This inverts colors and icons a bit, so they're not as harsh. So instead of a dark green item with white icon, its now light green with a dark green icon. I've also changed all text and comment boxes to be "grey" visually to separate out the UI from converation/actions. Give it a spin and let me know how this feels. I still need to update the comment UI.

Test Plan:
UIExamples, lots of various tasks and diffs.

{F1163837}

{F1163839}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15442