Add external link to Discord invite (#551)

Test Plan:
None really needed—the infrastructure has already been tested—but you
can verify that this works both under `yarn start` and `yarn build` by
navigating to the evident URL.

wchargin-branch: discord-invite

src/app/routeData.js

@@ -36,6 +36,15 @@ const routeData /*: $ReadOnlyArray<RouteDatum> */ = [
     title: "SourceCred explorer",
     navTitle: "Explorer",
   },
+  {
+    path: "/discord-invite",
+    contents: {
+      type: "EXTERNAL_REDIRECT",
+      redirectTo: "https://discord.gg/tsBTgc9",
+    },
+    title: "SourceCred Discord invite",
+    navTitle: null,
+  },
 ];
 exports.routeData = routeData;