Update Review Board for the new Djblets extension UI changes.
Review Request #11045 — Created June 17, 2020 and submitted
The new Djblets extension manager styling and template is built off of
more standard components, reducing the work we need to do in Review
Board. This change removes the custom styling and template, and adds a
setting to enable the "More Extensions" button.
Tested to make sure the extension manager still loads and works correctly.
Tested that the More Extensions button goes to the right place.