Refactor remove ActsAsTokenAuthenticationHandlerMethods

The behaviour that module used to add is part of
acting as a token authentication handler.
Keep private the token authentication handler interface

It makes sense for a token auhentication handler to be some kind
of authentication handler; one could therefore expect it to
implement the 'authentication handler' interface.

Implementing the authentication logic in a TokenAuthenticationHandler
class (versus module), would IMO make sense, but would require a major
renaming of public methods (starting by acts_as_token_authentication_hanlder).

Such a renaming could very probably lead to a better public interface
(e.g. setting `before_action :authenticate_user_from_token!` in
controllers, instead of the somewhat obscure token authentication
handling thing); however, it would _a priori_ require a major version
jump, which I don't want to perform now.

Keeping the interface private allows to decide later, and the main feature
of this branch --the interface specs-- are as useful being private
as being public.