Don't error on unverifie/unknownd devices.

When cross-signing is enabled, we no longer want to fail and
prompt the user to ack every device in the room. All the info should
be conveyed in the shield colour (although isn't fully just yet).

Fixes element-hq/element-web#11750
Requires matrix-org/matrix-js-sdk#1150

src/components/structures/MatrixChat.js

@@ -1506,6 +1506,15 @@ export default createReactClass({
             );
             cli.setGlobalBlacklistUnverifiedDevices(blacklistEnabled);
         }
+
+        // With cross-signing enabled, we send to unknown devices
+        // without prompting. Any bad-device status the user should
+        // be aware of will be signalled through the room shield
+        // changing colour. More advanced behaviour will come once
+        // we implement more settings.
+        cli.setGlobalErrorOnUnknownDevices(
+            !SettingsStore.isFeatureEnabled("feature_cross_signing")
+        );
     },
 
     showScreen: function(screen, params) {