Store API tokens encrypted in the database.
During the database migration a randomly generated key will be used encrypt the tokens if no ENCRYPTION_KEY environment variable was provided.
If the application is started without the presence of an ENCRYPTION_KEY (or if the key failed to decrypt the existing tokens), the UI will display a warning with further instructions.
* Allow to sign in with API tokens
* Test signing in with API tokens
* Add button to go back to login with credentials
* Add note about how to get API tokens