Misc CSS tweaks, timeline, mobile

Summary:
- Fixes line height when many long tasks are attached to a task.
- Tightens up mobile layout of timeline and object box
- Clean up aphront context bar

Test Plan: Tested all the changes, made tasks, stared at pixels.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3891

Differential Revision: https://secure.phabricator.com/D7169