MDL-59612 core: added previous and next links to activities

Part of MDL-59313.

theme/boost/templates/columns2.mustache

@@ -82,6 +82,7 @@
                     {{/hasregionmainsettingsmenu}}
                     {{{ output.course_content_header }}}
                     {{{ output.main_content }}}
+                    {{{ output.activity_navigation }}}
                     {{{ output.course_content_footer }}}
                     </div>
                 </section>