Allow editing of commit message from UI.

This change adds a small edit icon on the top right
corner of the commit message box on the change screen.

Clicking it lets the user edit the commit message and a
new patch set will be created.

Change-Id: I2c4d4a3ba520d4c12ade4bb3b47a55ce7f1b19df

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeScreen.java

@@ -281,6 +281,7 @@ public class ChangeScreen extends Screen
 
     descriptionBlock.display(detail.getChange(),
         detail.isStarred(),
+        detail.canEditCommitMessage(),
         detail.getCurrentPatchSetDetail().getInfo(),
         detail.getAccounts(), detail.getSubmitTypeRecord());
     dependsOn.display(detail.getDependsOn());