Allow review request names to wrap in admin lists.

Our admin styles had "text-wrap: nowrap" for <th> tags, which was meant for the
column headers, but the primary column (review requests summaries, for
example) also used th. This change fixes that style definition to only apply to
descendents of <thead>.

Testing done:
Created a review request with a really long summary and loaded
/admin/db/reviews/reviewrequest. Saw that things didn't look ugly.

Fixes bug 2125.

Reviewed at https://reviews.reviewboard.org/r/5402/