add default branding parameter for email notifs

config.json

@@ -1,6 +1,7 @@
 {
     "default_hs_url": "https://matrix.org",
     "default_is_url": "https://vector.im",
+    "brand": "Vector",
     "integrations_ui_url": "http://localhost:8081/",
     "integrations_rest_url": "http://localhost:5050"
 }