Switch to new OAuth server

[jirutka]

We are slowly migrating to new instance of OAuth server. The old version is still in use, but I’m gonna switch it off soon. There are some (simple) changes that you should done:

• Change URL of the authorization endpoint and the token endpoint:
  • https://auth.fit.cvut.cz/oauth/oauth/authorize → https://auth.fit.cvut.cz/oauth/authorize (fitauth:7),
  • https://auth.fit.cvut.cz/oauth/oauth/token → https://auth.fit.cvut.cz/oauth/token (fitauth:8).
• Make that social.backends.oauth passes client_id and client_secret using Authorization header (as Authorization: Basic base64(<client_id>:<client_secret>)) and not in the request’s body (it’s not recommended by RFC and allowed anymore); see this page for more information.
• Use new userinfo endpoint instead of tokeninfo endpoint to obtain username of the user who authorized the token:
  • https://auth.fit.cvut.cz/oauth/api/v1/tokeninfo?token=<access_token> → https://auth.fit.cvut.cz/oauth/userinfo (Authorization: Bearer <access_token>) (fitauth:22),
  • This endpoint returns JSON with username in the attribute username (instead of user_id) (fitauth:10).