ChangeScreen2: allow edit commit message

Add new "Edit Message" button in change header and bind it to "e"
shortcut.  Add a new UiAction as REST API endpoint to create a new
revision with updated commit message.  Reload the screen on success.

Change-Id: Ic555f16efba4783eca9684340b27f9d58b1482bb

gerrit-gwtui/src/main/java/com/google/gerrit/client/change/Resources.java

@@ -33,5 +33,6 @@ public interface Resources extends ClientBundle {
     String button();
     String popup();
     String popupContent();
+    String section();
   }
 }