User entity: simplify
InMemoryAuthenticationService
UserRegistrationUseCase
Refactored the test for FirebaseDataService

...to not rely on the global XO.authenticationService because it’ll no
loger be a global: it will be a main-local and will be injected where
needed. This is to prevent the chances of global-coupling.
Registration form

TODO: fix the registration promise to:
- return the password on success
- replace Firebase error messages with user friendlier messages