CSS tweaks to mobile maniphest.

Summary: Some minor cleanup, remove preview, widen transactions, remove timestamps (i could go either way). I mainly want to interact more on mobile but am finding its pretty crowded. I still need to think more about these views.

Test Plan: Review on iOS and Chrome

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4680