Allow ChangeEmail derived classes to append to the mail footer

Add a new protected method, `formatFooter()`, on the `ChangeMail`
class.  Derived classes can override this method to append lines
on to the footer of change-related mails.

This will allow different footer lines to be appended depending
on the type of change mail.

Change-Id: Ia0694ad83fc435dd01e65240c341ffee1f553e44

gerrit-server/src/main/java/com/google/gerrit/server/mail/ChangeEmail.java

@@ -121,11 +121,16 @@ public abstract class ChangeEmail extends OutgoingEmail {
       }
     } catch (OrmException e) {
     }
+    formatFooter();
   }
 
   /** Format the message body by calling {@link #appendText(String)}. */
   protected abstract void formatChange() throws EmailException;
 
+  /** Format the message footer by calling {@link #appendText(String)}. */
+  protected void formatFooter() throws EmailException {
+  }
+
   /** Setup the message headers and envelope (TO, CC, BCC). */
   protected void init() throws EmailException {
     if (args.projectCache != null) {