Add MessageOfTheDay extension point for plugins

Allow plugins to contribute messages to the UI during initial page
load. Gerrit displays the messages in a butter bar at the top of
the page and allows users to hide them with the "Dismiss" button.

Messages are hidden per-browser using a cookie named after the
message id, set to expire at a date supplied by the server. After
this date the same message could redisplay if the server sends the
same message again.

Change-Id: I0bcca845f501cbeb8c31356fff398c3adb43099a

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

@@ -565,6 +565,9 @@ public class Gerrit implements EntryPoint {
     }
 
     saveDefaultTheme();
+    if (hpd.messages != null) {
+      new MessageOfTheDayBar(hpd.messages).show();
+    }
     PluginLoader.load(hpd.plugins, token);
   }