handle device / client absence for deeplinks

Summary:
This diff makes the new deeplink format feature complete, make sure VPN connection, plugin installation, client & device selection are now all handled. See the test plan for examples.

Changelog: Flipper now supports a richer protocol for opening deeplinks: https://fbflipper.com/docs/extending/deeplinks#open-plugin

Reviewed By: timur-valiev

Differential Revision: D30423809

fbshipit-source-id: e6cf4bf852b2c64e9a79a33ef0842eb27f68f840

desktop/flipper-plugin/src/utils/renderReactRoot.tsx

@@ -17,6 +17,7 @@ import {render, unmountComponentAtNode} from 'react-dom';
 export function renderReactRoot(
   handler: (unmount: () => void) => React.ReactElement,
 ): () => void {
+  // TODO: find a way to make this visible in unit tests as well
   const div = document.body.appendChild(document.createElement('div'));
   const unmount = () => {
     unmountComponentAtNode(div);