Clean up authentication code

[Glutexo]

The current authentication code is a mess. It contains some remnants from previous suboptimal solutions and generally lacks any clear semantics. Some specific problems I see there are:

• In the app.auth module:
  • Two authentication functions (authentication_header_handler, _ bearer_token_handler_) that differ only in one one call (from_auth_header vs. from_bearer_token – this can be DRIED.
  • Catching all Exceptions (both in authentication_header_handler and in bearer_token_handler) instead of just the really expected one – this can lead to silent failures instead of correctly crashing.
• In the app.auth.identity module:
  • The Identity class serves two puproses – storing the account number (or None), which is needed in the operations, and storing the bearer token, which is not. We can either only store an account number in it, or get rid of the class completely and use the sole account number as an identity.
  • The _asdict and eq methods of the Identity class are unnecessary and can be removed.
  • The validate method validates either account number presence or bearer token validity. This can be moved to the validation handlers in app.auth.
  • An environment variable with the shared secred is loaded there, but should be in the app.config module instead. This is already acknowledged.