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
that would requre returning from `get_user` not unly user but request_field where it was found, for better error log if it does not match. IMHO not worth it.