Pop the buttons

Summary: Keeping in step with the gradients on the grey buttons, this adds slight gradients and hover states to blue and green. I feel like they are much more obviously buttons now in the UI (on headers for example).

Test Plan:
Review UI Buttons, check differential, actions, other random buttons.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15379