This also changes the way we treat Blender ID tokens. Before, the Blender ID
token was discarded and a random token was generated & stored. Now the
actual Blender ID token is stored.
The Facebook and Google OAuth code still uses the old approach of generating
a new token. Not sure what the added value is, though, because once the
Django session is gone there is nothing left to authenticate the user and
thus the random token is useless anyway.
Now BLENDER_ID_ENDPOINT is used for the Blender ID OAuth config,
and it's directly accessed when building requests for Blender ID token
validation (without using utility functions).
Also added SERVER_NAME in config_testing and pre-populated the keys of OAUTH_CREDENTIALS, since the implementation of providers is part of the application.
