[backend,accounts] Move signal registration from backend to accounts app where it should be

beat/web/accounts/apps.py

@@ -37,3 +37,4 @@ class AccountsConfig(AppConfig):
 
     def ready(self):
         registry.register(User)
+        from .signals import setup_user

beat/web/backend/apps.py

@@ -32,6 +32,3 @@ class BackendConfig(AppConfig):
 
     name = 'beat.web.backend'
     verbose_name = _('Backend')
-
-    def ready(self):
-        from .signals import setup_user