FIX: set showFooter to true on group topic list (#8795)

If using {{#if showFooter}} in a template, showFooter is never set to true on a group's g/groupname/activity/topics route (it's correctly set on other group routes like group-activity-posts)

app/assets/javascripts/discourse/routes/group-activity-topics.js.es6

@@ -1,6 +1,8 @@
 import DiscourseRoute from "discourse/routes/discourse";
 
 export default DiscourseRoute.extend({
+  showFooter: true,
+
   titleToken() {
     return I18n.t(`groups.topics`);
   },