Prepare for Django 2.0

Prepare for Django 2.0

Two changes necessary.

- Remove 'SessionAuthenticationMiddleware' from settings

  https://docs.djangoproject.com/en/2.0/releases/2.0/#miscellaneous

- Replace 'render_to_response' with 'render'

  https://docs.djangoproject.com/en/2.0/releases/2.0/#miscellaneous

Both of these are supported by Django 1.11 so there's no need for compat
wrappers.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Daniel Axtens <dja@axtens.net>