Move over from rolling custom media library code to the Media modal code.

A few things to note here.

The easiest workflow for modifying Backbone code here without pulling our hair out is overriding the core versions and creating patched versions. We can regularly rebase off of core to make for merge-ability.

I took out the JS code that was powering the previous implementation. Now that we're moving towards using the WP media, we can of course re-implement any features that were in that version, but I'll hold off implementing any of these for the moment pending discussion.