JWT decoder middleware

[pedro-gutierrez]

This PR re-visits our auth mechanism and replaces the existing cognito based logic by a more generic JWT approach that is auth0 compatible.

• [x] We now use Joken in for JWT encoding/decoding, instead of Guardian.
• [x] Cognito dependency has been removed.
• [x] Provided auth tests are end-to-end (no mocks).

Fixes: #96, #18