Refactor extract FallbackAuthenticationHandler, inject dependency

Add FallbackAuthenticationHandler private specs (there is no public
interface for now)

The dependency on the Devise authentication helpers is now encapsulated in
the `ActsAsTokenAuthenticationHandler#fallback_authentication_handler`
method.

If necessary, the `#authenticate_entity_from_fallback!` specs could
be written using a DummyAuthenticationHandler instead of
the Devise-dependent FallbackAuthenticationHandler.
Minor refactor rename variables for improved readability
Refactor extract Entity to encapsulate entity_* methods