Add Epics in issuable list when EE

Create a method in LabelsHelper which returns the correct
list of `issuable_types` to the view.

app/helpers/labels_helper.rb

@@ -276,6 +276,10 @@ module LabelsHelper
     klass.new(hash.slice(:color, :description, :title, :group_id, :project_id))
   end
+  def issuable_types
+    ['issues', 'merge requests']
+  end
+
   # Required for Banzai::Filter::LabelReferenceFilter
   module_function :render_colored_label, :text_color_for_bg, :escape_once, :label_tooltip_title
 end

app/views/groups/labels/index.html.haml

 - @no_container = true
-- page_title "Labels"
+- page_title 'Labels'
 - can_admin_label = can?(current_user, :admin_label, @group)
-- issuables = ['issues', 'merge requests']
 - search = params[:search]
 - subscribed = params[:subscribed]
 - labels_or_filters = @labels.exists? || search.present? || subscribed.present?
@@ -14,7 +13,7 @@
     .labels-container.prepend-top-5
       - if @labels.any?
         .text-muted
-          = _('Labels can be applied to %{features}. Group labels are available for any project within the group.') % { features: issuables.to_sentence }
+          = _('Labels can be applied to %{features}. Group labels are available for any project within the group.') % { features: issuable_types.to_sentence }
         .other-labels
           %h5= _('Labels')
           %ul.content-list.manage-labels-list.js-other-labels