Support Django-1.8

Django < 1.8 is not supported any more by upstream, and Horizon has dropped django 1.7 and lower version support.

RedirectView.permanent` change:
https://docs.djangoproject.com/en/1.8/ref/class-based-views/base/

`url` change:
https://docs.djangoproject.com/en/1.8/internals/deprecation/
See version 1.5 notes for the url and ssi template tag change

`django.forms.utils` change:
https://docs.djangoproject.com/en/1.8/internals/deprecation/
See version 1.9 notes

`firstof` change:
https://docs.djangoproject.com/en/1.8/internals/deprecation/
See version 1.8 notes for cycle and firstof template tags