TNL-782 each request should have valid ID Token

Each incoming request is expected to have signed non-expired JSON Web Token, containing information about user (sub), matching edx-notes client id (to be set in provider's database) and signed with edx-notes client secret (the same). 

https://openedx.atlassian.net/browse/TNL-782
trivial: if `token` defaults to `None`, and you only set credentials when it is not None, you could also use this helper in `test_no_token`