Support handling deeplinks in plugins

Summary:
This adds support for handling incoming deeplinks in a Sandy plugin, which can be done by using a `client.onDeepLink(deepLink => { } )` listener

Also generalized deeplinks to not just support strings, but also richer objects, which is beneficial to plugin to plugin linking.

Reviewed By: jknoxville

Differential Revision: D22524749

fbshipit-source-id: 2cbe8d52f6eac91a1c1c8c8494706952920b9181

desktop/app/src/NotificationsHub.tsx

@@ -55,7 +55,7 @@ type DispatchFromProps = {
   selectPlugin: (payload: {
     selectedPlugin: string | null;
     selectedApp: string | null;
-    deepLinkPayload: string | null;
+    deepLinkPayload: unknown;
   }) => any;
   updatePluginBlacklist: (blacklist: Array<string>) => any;
   updateCategoryBlacklist: (blacklist: Array<string>) => any;
@@ -414,7 +414,7 @@ type ItemProps = {
   selectPlugin?: (payload: {
     selectedPlugin: string | null;
     selectedApp: string | null;
-    deepLinkPayload: string | null;
+    deepLinkPayload: unknown;
   }) => any;
   logger?: Logger;
   plugin: PluginDefinition | null | undefined;