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
I thought we should add http://www.django-rest-framework.org/api-guide/authentication/#custom-authentication and use default https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/permissions.py#L41 then.