Rework sidebar selection

Summary:
- Make sure newly connecting apps are automatically selected
- Improved the sidebar UI by using more consistent, spacious styling, and giving some more attention to error states

Reviewed By: passy

Differential Revision: D18505636

fbshipit-source-id: 18b2c8e78be13aabb3a54c60553f6b0d1e613b27

headless/index.tsx

@@ -314,6 +314,9 @@ async function startFlipper(userArguments: UserArguments) {
             const ports = store.getState().application.serverPorts;
             matchedDevice.reverse([ports.secure, ports.insecure]);
           }
+          matchedDevice.loadDevicePlugins(
+            store.getState().plugins.devicePlugins,
+          );
           store.dispatch({
             type: 'REGISTER_DEVICE',
             payload: matchedDevice,