Move all EE helper differences to EE specific modules

EE specific code that resides in app/helpers should be moved to the corresponding EE specific modules that reside in ee/app/helpers, leaving behind only the necessary prepend and include calls, which should be placed at the end of the file).

Differences

app/helpers/selects_helper.rb

diff --git a/home/yorickpeterse/Projects/gitlab/gdk-ce/gitlab/app/helpers/selects_helper.rb b/home/yorickpeterse/Projects/gitlab/gdk-ee/gitlab/app/helpers/selects_helper.rb
index 2f802e4eab8..d28eb5d87cf 100644
--- a/home/yorickpeterse/Projects/gitlab/gdk-ce/gitlab/app/helpers/selects_helper.rb
+++ b/home/yorickpeterse/Projects/gitlab/gdk-ee/gitlab/app/helpers/selects_helper.rb
@@ -28,6 +28,7 @@ module SelectsHelper
   def groups_select_tag(id, opts = {})
     classes = Array.wrap(opts[:class])
     classes << 'ajax-groups-select'
+    classes << 'multiselect' if opts[:multiple]
 
     # EE requires this line to be present, but there is no easy way of injecting
     # this into EE without causing merge conflicts. Given this line is very
@@ -70,7 +71,6 @@ module SelectsHelper
 
     opts[:class] = klass_opts.join(' ')
     value = opts[:selected] || ''
-
     hidden_field_tag(id, value, opts)
   end
 
@@ -89,3 +89,5 @@ module SelectsHelper
     }
   end
 end
+
+SelectsHelper.prepend(EE::SelectsHelper)

app/helpers/sorting_helper.rb

diff --git a/home/yorickpeterse/Projects/gitlab/gdk-ce/gitlab/app/helpers/sorting_helper.rb b/home/yorickpeterse/Projects/gitlab/gdk-ee/gitlab/app/helpers/sorting_helper.rb
index 67c808b167a..4a1cc81b3bd 100644
--- a/home/yorickpeterse/Projects/gitlab/gdk-ce/gitlab/app/helpers/sorting_helper.rb
+++ b/home/yorickpeterse/Projects/gitlab/gdk-ee/gitlab/app/helpers/sorting_helper.rb
@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 module SortingHelper
+  prepend ::EE::SortingHelper
+
   def sort_options_hash
     {
       sort_value_created_date     => sort_title_created_date,
@@ -164,7 +166,7 @@ module SortingHelper
     reverse_sort = issuable_reverse_sort_order_hash[sort_value]
 
     if reverse_sort
-      reverse_url = page_filter_path(sort: reverse_sort, label: true)
+      reverse_url = page_filter_path(sort: reverse_sort)
     else
       reverse_url = '#'
       link_class += ' disabled'