Make timeline view prettier

Summary: Aligns the timeline view more closely with the diff_full_view.png mock.

Test Plan:
Desktop:

{F26822}

Mobile:

{F26823}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T2104

Differential Revision: https://secure.phabricator.com/D4137