Change Forum navbar item to Questions

The projects navbar should mirror the one on the spring.io website.

_config.yml

@@ -34,4 +34,4 @@ custom_gradle_template: false
 
 main_site_url: http://spring.io
 projects_site_url: http://projects.spring.io
-forum_url: http://forum.spring.io
+questions_url: http://spring.io/questions

_includes/page.html

@@ -72,7 +72,7 @@
                         href="{{site.main_site_url}}/projects">Projects</a></li>
                     <li class="navbar-link"><a href="{{site.main_site_url}}/blog">Blog</a>
                     </li>
-                    <li class="navbar-link"><a href="{{site.forum_url}}">Forum</a>
+                    <li class="navbar-link"><a href="{{site.questions_url}}">Questions</a>
                     </li>
                     <li class="navbar-link nav-search"><i
                         class="icon-search navbar-search--icon js-search-input-open"></i>