Scaffold for user's logged-in dashboard

This change introduces a dependency of the routing behavior on
whether the user is logged in. Since the WebComponentsReady event
can fire before OR after the account info is retrieved, a Promise
is used to ensure deterministic ordering via `accountReady`.

There is also some minor cleanup of which element is being used
as the global `app` object.

Feature: Issue 3700

Change-Id: I9768f2eabd5cdb7c62ead16ae97df5c1c321eaf6

polygerrit-ui/server.go

@@ -101,7 +101,7 @@ type server struct{}
 
 // Any path prefixes that should resolve to index.html.
 var (
-	fePaths    = []string{"/q/", "/c/"}
+	fePaths    = []string{"/q/", "/c/", "/dashboard/"}
 	issueNumRE = regexp.MustCompile(`^\/\d+\/?$`)
 )