Add a config option to block all room invites (#2457)

- allows sysadmins the ability to lock down their servers so that people can't
send their users room invites.

tests/utils.py

@@ -56,6 +56,7 @@ def setup_test_homeserver(name="test", datastore=None, config=None, **kargs):
         config.worker_replication_url = ""
         config.worker_app = None
         config.email_enable_notifs = False
+        config.block_non_admin_invites = False
 
     config.use_frozen_dicts = True
     config.database_config = {"name": "sqlite3"}