Allow document search within post edit screen

[eyeseast]

It would be awesome if I could embed documents in a post without ever having to leave WP. We should be able to do this entirely with javascript. Backbone and Underscore are built into WP now, so we don't need any added dependencies.

I've built something like this before (https://github.com/sunlightlabs/navis-openstates) so it shouldn't be too hard.