Add `categories-list` css class to body for customization purposes

app/assets/javascripts/discourse/views/discovery-categories.js.es6

@@ -1,3 +1,11 @@
 import UrlRefresh from 'discourse/mixins/url-refresh';
 
-export default Discourse.View.extend(UrlRefresh, Discourse.ScrollTop);
+export default Discourse.View.extend(UrlRefresh, Discourse.ScrollTop, {
+  _addBodyClass: function() {
+    $('body').addClass('categories-list');
+  }.on('didInsertElement'),
+
+  _removeBodyClass: function() {
+    $('body').removeClass('categories-list');
+  }.on('willDestroyElement')
+});

test/javascripts/integration/topic-discovery-test.js.es6

@@ -11,15 +11,20 @@ test("Visit Discovery Pages", function() {
   andThen(function() {
     ok(exists(".topic-list"), "The list of topics was rendered");
     ok(exists('.topic-list .topic-list-item'), "has topics");
+    ok($('body.category-1').length, "has a custom css class for the category id on the body");
   });
 
   visit("/categories");
   andThen(function() {
+    ok($('body.category-1').length === 0, "removes the custom category class");
+
     ok(exists('.category'), "has a list of categories");
+    ok($('body.categories-list').length, "has a custom class to indicate categories");
   });
 
   visit("/top");
   andThen(function() {
+    ok($('body.categories-list').length === 0, "removes the `categories-list` class");
     ok(exists('.topic-list .topic-list-item'), "has topics");
   });
 });