Add config form styles specific to Review Board that were in Djblets.
Review Request #1637 — Created April 10, 2019 and submitted
When the config forms stylesheet was moved into Djblets, some styles specific to Review Board remained. As part of a major revamp of the stylesheets, some of these styles are scheduled to be removed. This change prepares for that by moving those styles back into Review Board, and makes use of the config forms support throughout the Django admin UI. Testing Done: Tested that the My Account page looked as it did before after the removal of these styles in Djblets, and that pages in the Admin UI using the new config forms styles looked right. Reviewed at https://reviews.reviewboard.org/r/10433/