Added django-admin-bootstrapped as dependency

## Motivation

The front-end for GeoNode now has had a major face-lift and uses Bootstrap to provide clear styling.  However the Django admin still carries a look-and-feel that is out of date in modern web apps.  There are a number of third-party apps for styling the admin, but django-admin-bootstrapped is arguably one of the most straightforward and effective, and the look-and-feel is comparable to that of the "new" GeoNode.