Rework theme selection entirely

[zerocrates]

The problems with the Theme dropdown and its Configure button have been around for as long as Exhibit Builder's had that feature, with issues such as #40 (which is really just a band-aid). These issues have been re-raised in testing recently.

Along with that, we have much older issues like #3. It seems like we just need an entirely updated interface for choosing themes, perhaps one that more closely mirrors the interface for choosing themes for the whole site.

This will need @kimisgold and probably input from @sheilabrennan