Use custom class to select Epic page wrapper

ee/app/assets/javascripts/epic/epic_tabs.js

@@ -5,7 +5,7 @@ import initRelatedItemsTree from 'ee/related_items_tree/related_items_tree_bundl
 export default class EpicTabs {
   constructor() {
     this.epicTreesEnabled = gon.features && gon.features.epicTrees;
-    this.wrapper = document.querySelector('.content-wrapper .container-fluid:not(.breadcrumbs)');
+    this.wrapper = document.querySelector('.js-epic-container:not(.breadcrumbs)');
     this.epicTabs = this.wrapper.querySelector('.js-epic-tabs-container');
     this.discussionFilterContainer = this.epicTabs.querySelector('.js-discussion-filter-container');
     const allowSubEpics = parseBoolean(this.epicTabs.dataset.allowSubEpics);

ee/app/views/groups/epics/show.html.haml

 - @no_breadcrumb_container = false
 - @no_container = false
-- @content_class = "limit-container-width" unless fluid_layout
+- @content_class = "js-epic-container #{'limit-container-width' unless fluid_layout}"
 
 - epic_reference = @epic.to_reference
 - sub_epics_feature_available = @group.feature_available?(:subepics)