Flatten Object List items and lighten up grey buttons

Summary: Moving towards a flatter, crisper layout with some minor tweaks here. The button styles and object item styles now have consistent colors and depth. Will continue to repeat this pattern and build out a standard 'grey' palette.

Test Plan: Test homepage and maniphest home.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6828