Add models for manual invoicing

Advances #2

aleksis/apps/tezor/apps.py

@@ -18,9 +18,7 @@ class DefaultConfig(AppConfig):
     def ready(self):
         from django.conf import settings  # noqa
         settings.PAYMENT_MODEL = "tezor.Invoice"
-        settings.PAYMENT_VARIANTS = {
-            "dummy": ("payments.dummy.DummyProvider", {})
-        }
+        settings.PAYMENT_VARIANTS = {}
 
         for app_config in apps.app_configs.values():
             if hasattr(app_config, "get_payment_variants"):