This change adds dark mode styling (and generally overrides the default Selectize view) for the related object selector. This isn't something that's currently visible in Review Board, since the admin interface is always in light mode, but it helps in RBCommons or other projects using Djblets. Also, I hadn't noticed that the option rendering change to allow subclasses to use a table for the selected options broke the selectize drop-down. I've changed it so there are now separate functions for the drop-down and selected options (the default implementation of the former just calling the former), so that we can provide a separate layout for the drop-down and the selected items. Testing Done: - Verified that both the drop-down and selected items lists looked correct for both <table> and <li> based selectors. - Verified that the selectors looked good in both light and dark modes.