Tweak maniphest css to make width of task transactions consistent with width of task metadata panel.

Summary: see title.

Test Plan: it looks good! also increased and decreased the font-size and
verified the width remained consistent. did this on chrome and firefox on a
mac.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1494