PLT-6882 Disable /invite_people when account creation is set to false (#6709)

* PLT-6882 Disable /invite_people when account creation is set to false

* Disable /invite_people when account creation or email notifications are set to false

i18n/en.json

@@ -451,6 +451,10 @@
     "id": "api.command.invite_people.email_off",
     "translation": "Email has not been configured, no invite(s) sent"
   },
+  {
+    "id": "api.command.invite_people.invite_off",
+    "translation": "User creation has been disabled on this server, no invite(s) sent"
+  },
   {
     "id": "api.command.invite_people.fail",
     "translation": "Encountered an error sending email invite(s)"