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.

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.