Agency admin: batch upload ability

[mtigas]

Depends on #9. Probably targeting summer beta, not prototype.

After we replace the django admin and use a customized admin, we set up a special version of the "admin facing 'add file'" page, maybe using a FormSet to allow uploading ten files at a time or something.

Alternatively, we allow the upload of .zip files, in which case we add the zip file itself as a Document, then unzip it and loop over every individual file and add each as a Document, too.