Fix margins on the change description summary field in changedesc entries.

Review Request #142 — Created Nov. 9, 2016 and updated — Latest diff uploaded

Review Board
The recent CSS cleanup introduced a few small regressions, one of which was the
margins on the "Change Summary" field in change description entries on the
review request page. This change fixes those up.

Testing done:
Published an update including a summary, and saw that the layout looked the
same as similar multiline text fields within the review request box.

Reviewed at