Remove Django deprecation warning

[stsewd]

The warning was

The standalone TEMPLATE_* settings were deprecated in Django 1.8 and the TEMPLATES dictionary takes precedence. You must put the values of the following settings into your default
TEMPLATES dict: TEMPLATE_DEBUG.

And we already have the debug option there

https://github.com/rtfd/readthedocs.org/blob/b05c82acc69e492533da9b7e3e40fef2456af6de/readthedocs/settings/base.py#L176

Also, there is a warning about Guardian

Guardian authentication backend is not hooked. You can add this in settings as eg: AUTHENTICATION_BACKENDS = ('django.contrib.auth.backends.ModelBackend', 'guardian.backends.ObjectPermissionBackend').

I think that setting is used in production, right?

[davidfischer]

Firstly, I'm happy you're taking care of this. It has been on my list for a while. This is safe to remove and the setting we have in the TEMPLATES dict is sufficient. This PR is good to go!

I believe there is a small issue in our production settings, however. This change didn't introduce it. In our production settings that we have TEMPLATE_DEBUG=False but the prod settings file doesn't change settings.TEMPLATES[0]['OPTIONS']['debug'] which I believe it should. I'll open a PR for that.