Added a class name to the powered by text

no issues

- our other themes have this class, and this makes it consistent across over all offcial themes

default.hbs

@@ -80,7 +80,7 @@
             <nav class="site-footer-nav">
                 {{navigation type="secondary"}}
             </nav>
-            <div><a href="https://ghost.org/" target="_blank" rel="noopener">Powered by Ghost</a></div>
+            <div class="gh-powered-by"><a href="https://ghost.org/" target="_blank" rel="noopener">Powered by Ghost</a></div>
         </div>
     </footer>