(fix) URLs containing networks with slashes are now handled properly.

Networks given as IP addresses with masks (e.g. `10.24.1.0/24`) were giving 404 errors, since there were no url patterns for such cases. The following PR fixes this.

Unfortunately, Django replaces `%2F` with `/` - hence those more "elaborate" patterns in `urls.py` (relevant discussion: https://code.djangoproject.com/ticket/15718).