Add "New epic" item to "New (+)" dropdown

This item is only enabled if the current user can create epics and it's
under the create_epic_form flag

app/views/layouts/header/_new_dropdown.haml

@@ -15,6 +15,7 @@
             %li= link_to _('New project'), new_project_path(namespace_id: @group.id)
           - if create_group_subgroup
             %li= link_to _('New subgroup'), new_group_path(parent_id: @group.id)
+          = render_if_exists 'layouts/header/create_epic_new_dropdown_item'
           %li.divider
           %li.dropdown-bold-header GitLab
 

ee/app/controllers/ee/groups_controller.rb

@@ -9,6 +9,10 @@ module EE
       alias_method :ee_authorize_admin_group!, :authorize_admin_group!
 
       before_action :ee_authorize_admin_group!, only: [:restore]
+
+      before_action do
+        push_frontend_feature_flag(:create_epic_form, @group)
+      end
     end
 
     override :render_show_html

ee/app/views/layouts/header/_create_epic_new_dropdown_item.haml

+- create_group_epic = can?(current_user, :create_epic, @group)
+- return unless create_group_epic && Feature.enabled?(:create_epic_form)
+
+%li= link_to _('New epic'), new_group_epic_path(@group)

ee/spec/views/layouts/header/_new_dropdown.haml_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'layouts/header/_new_dropdown' do
+  let(:user) { create(:user) }
+
+  context 'group-specific links' do
+    let(:group) { create(:group) }
+
+    before do
+      allow(view).to receive(:current_user).and_return(user)
+
+      assign(:group, group)
+    end
+
+    context 'as a Group owner' do
+      before do
+        group.add_owner(user)
+      end
+
+      it 'does not have "New epic" link' do
+        render
+
+        expect(rendered).not_to have_link(
+          'New epic',
+          href: new_group_epic_path(group)
+        )
+      end
+
+      context 'when epics are enabled' do
+        before do
+          stub_licensed_features(epics: true)
+          stub_feature_flags(create_epic_form: true)
+        end
+
+        it 'has a "New epic" link' do
+          render
+
+          expect(rendered).to have_link(
+            'New epic',
+            href: new_group_epic_path(group)
+          )
+        end
+      end
+    end
+  end
+end