Tighten up CSS on object-list a bit.

Summary:
- Fix unintended round edges.
- Reduce height 4px;
- Align title and date
- Fix mobile/tablet spacing

Test Plan: Photoshop, UIExamples, Config, Paste, Maniphests (all mobile and desktop views)

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5445