Update Timeline for Tablet/Mobile

Summary: Minor CSS fixes Fixes T4419

Test Plan: Test phone and table breakpoints, submit a new comment.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4419

Differential Revision: https://secure.phabricator.com/D8211