Add setting to allow MLC registration only (#1259)

* Adds a setting to registration visibility to allow for MLC registration while registration is disabled

CTFd/utils/config/integrations.py

@@ -3,3 +3,8 @@ from CTFd.utils import get_config
 
 def mlc():
     return get_config("oauth_client_id") and get_config("oauth_client_secret")
+
+
+def mlc_registration():
+    v = get_config("registration_visibility")
+    return v == "mlc"

CTFd/utils/config/visibility.py

@@ -40,3 +40,5 @@ def registration_visible():
         return True
     elif v == "private":
         return False
+    else:
+        return False