Add a loading message, link to the project, before the UI initializes

It can take a second or two for the GWT code to download, parse, and
start executing.  During that time its a good idea to give the user
more than just a blank window to look at, so we embed a very basic
loading message into the default DOM in the host page.  In practice
it is hard to see this message, as the UI does start up pretty quick.
Once the UI is ready to install the first screen we drop the elements
out of the DOM tree, so the browser no longer needs to manage them.

We also embed a hyperlink back to the project homepage.  This can
be useful for non-JavaScript enabled browsers, so they can at least
jump back to the Gerrit project and learn more about why JavaScript
might need to be enabled here.  It also will help to boost our
project's rankings in search engines, as any search engine will
most likely index only the host page.

Anyone who is using Gerrit on their site should also help drive
traffic back to the main project, to promote more developers to get
interested in contributing.  It is in everyone's best interest if
we have a vibrant developer community behind Gerrit.

Signed-off-by: Shawn O. Pearce <sop@google.com>

src/main/java/com/google/gerrit/client/Gerrit.java

@@ -278,6 +278,9 @@ public class Gerrit implements EntryPoint {
   }
 
   private void showInitialScreen() {
+    final RootPanel sg = RootPanel.get("gerrit_startinggerrit");
+    sg.getElement().getParentElement().removeChild(sg.getElement());
+
     History.addHistoryListener(new Link());
     if ("".equals(History.getToken())) {
       if (isSignedIn()) {

src/main/java/com/google/gerrit/public/Gerrit.html

@@ -11,9 +11,12 @@
     <div id="gerrit_topmenu"></div>
     <div id="gerrit_endtopmenu"></div>
     <div id="gerrit_header"></div>
-    <noscript>
-      <p>Gerrit requires a JavaScript enabled browser.</p>
-    </noscript>
+    <div id="gerrit_startinggerrit" style="margin-left: 10px;">
+      <p>Loading <a href="http://code.google.com/p/gerrit/" target="_blank">Gerrit Code Review</a> ...</p>
+      <noscript>
+        <p>Gerrit requires a JavaScript enabled browser.</p>
+      </noscript>
+    </div>
     <div id="gerrit_body"></div>
     <div id="gerrit_pagefooter">
       <div id="gerrit_footer"></div>