Moved everything under `authentication.pages` to `authentication` and moved the API controller to `authentication.api`.
Simplified service-loading code.
Moved some signup-related methods from UserAccountService to SignupService.
Moved more signuo stuff from UserController to SignupController.
Moved more password reset stuff from UserAccountService to PasswordResetService.
Introduced `RequestHelpers.withQueryString`, to invoke form-based controller actions directly from Pless tests (without having to start a Play application).