Use a template to set the subject line.

Add an admin editable ChangeSubject.vm template used to
format the subject header in change emails.

Change-Id: Iea58807b9a947bf0a4eba31c04977582430137ed

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

@@ -40,7 +40,7 @@ public class CreateChangeSender extends NewChangeSender {
   }
 
   @Override
-  protected void init() {
+  protected void init() throws EmailException {
     super.init();
 
     bccWatchers();