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-780
Does it make sense to raise `ImproperlyConfigured` exception if `settings.CLIENT_ID` , `settings.CLIENT_SECRET` were not defined for some reasons?