Ensure filtering menu is shown when viewing review requests in the admin section

In the URL /admin/db/reviews/reviewrequest/ when you had a small number of
review requests, the filtering menu was not fully shown. I changed the minimum
height for the box displaying the review requests, thus also showing the
filtering menu completely.

Testing Done:
I checked that the UI looks fine on the Chrome and Firefox versions mentioned
in the issue.

Bugs closed: 3222

Reviewed at https://reviews.reviewboard.org/r/5433/