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'd like to move this code in helper method:
```python
def get_user(self, request):
  for request_field in ('GET', 'POST', 'DATA'):
    data = getattr(request, request_field)
    if 'user' in data:
      return data['user']
  return None
```
What do you think?