Add django-filebrowser to dependencies

This is a 3rd party extension to the Django admin interface which
allows the uploading of files to an arbitrary range of directories
on the server (e.g. /media/*).  I propose that we use it to allow
admins to modify their images and styles without the need for file
sharing.
Add filebrowser as an installed app with URLs
Add links to the filebrowser

For now, put them in templates wherever there are links to the admin
interface (because filebrowser is not directly linked from the admin
interface).
Merge pull request #848 from learning-unlimited/media-filebrowser

Use of django-filebrowser to allow static file uploads