Add helpers to reverse issuable sorting

this is just me testing to see where the linebreak should be because I d
- `issuable_sort_option_overrides` has sort options that are only
  available by reversing the sorting.
- `issuable_reverse_sort_order_hash` maps sorting in one direction
  to the other, both ways.
- `issuable_sort_option_title` uses `issuable_sort_option_overrides` to
  get the correct title (so you see the same sort title no matter the
  direction we ordered in).
- `issuable_sort_direction_button` shows a button to reverse the
  sorting. It's disabled if the current sort is irreversible
  (like for Priority).

app/helpers/sorting_helper.rb

@@ -138,6 +138,56 @@ module SortingHelper
     link_to item, path, class: sorted_by == item ? 'is-active' : ''
   end
 
+  def issuable_sort_option_overrides
+    {
+      sort_value_oldest_created => sort_value_created_date,
+      sort_value_oldest_updated => sort_value_recently_updated,
+      sort_value_milestone_later => sort_value_milestone
+    }
+  end
+
+  def issuable_reverse_sort_order_hash
+    {
+      sort_value_created_date => sort_value_oldest_created,
+      sort_value_recently_created => sort_value_oldest_created,
+      sort_value_recently_updated => sort_value_oldest_updated,
+      sort_value_milestone => sort_value_milestone_later
+    }.merge(issuable_sort_option_overrides)
+  end
+
+  def issuable_sort_option_title(sort_value)
+    sort_value = issuable_sort_option_overrides[sort_value] || sort_value
+
+    sort_options_hash[sort_value]
+  end
+
+  def issuable_sort_direction_button(sort_value)
+    link_class = 'btn btn-default'
+    reverse_sort = issuable_reverse_sort_order_hash[sort_value]
+
+    if reverse_sort
+      reverse_url = page_filter_path(sort: reverse_sort)
+    else
+      reverse_url = '#'
+      link_class << ' disabled'
+    end
+
+    link_to(reverse_url, type: 'button', class: link_class) do
+      icon_suffix =
+        if sort_value == sort_value_milestone
+          'lowest'
+        elsif sort_value == sort_value_due_date
+          'lowest'
+        elsif sort_value.ends_with?('_asc')
+          'lowest'
+        else
+          'highest'
+        end
+
+      sprite_icon("sort-#{icon_suffix}", size: 16)
+    end
+  end
+
   # Titles.
   def sort_title_access_level_asc
     s_('SortOptions|Access level, ascending')

app/views/shared/_sort_dropdown.html.haml

-- sorted_by = sort_options_hash[@sort]
+- sort_value = @sort
+- sort_title = issuable_sort_option_title(sort_value)
 - viewing_issues = controller.controller_name == 'issues' || controller.action_name == 'issues'
 
 .dropdown.inline.prepend-left-10
   .btn-group{ role: 'group' }
     .btn-group{ role: 'group' }
       %button.dropdown-toggle{ type: 'button', data: { toggle: 'dropdown', display: 'static' }, class: 'btn btn-default' }
-        = sorted_by
+        = sort_title
         = icon('chevron-down')
       %ul.dropdown-menu.dropdown-menu-right.dropdown-menu-selectable.dropdown-menu-sort
         %li
-          = sortable_item(sort_title_priority,         page_filter_path(sort: sort_value_priority,         label: true), sorted_by)
-          = sortable_item(sort_title_created_date,     page_filter_path(sort: sort_value_created_date,     label: true), sorted_by)
-          = sortable_item(sort_title_recently_updated, page_filter_path(sort: sort_value_recently_updated, label: true), sorted_by)
-          = sortable_item(sort_title_milestone,        page_filter_path(sort: sort_value_milestone,        label: true), sorted_by)
-          = sortable_item(sort_title_due_date,         page_filter_path(sort: sort_value_due_date,         label: true), sorted_by) if viewing_issues
-          = sortable_item(sort_title_popularity,       page_filter_path(sort: sort_value_popularity,       label: true), sorted_by)
-          = sortable_item(sort_title_label_priority,   page_filter_path(sort: sort_value_label_priority,   label: true), sorted_by)
-    %a{ type: 'button', href: '#', class: 'btn btn-default' }
-      = sprite_icon('sort-lowest', size: 16)
+          = sortable_item(sort_title_priority,         page_filter_path(sort: sort_value_priority,         label: true), sort_title)
+          = sortable_item(sort_title_created_date,     page_filter_path(sort: sort_value_created_date,     label: true), sort_title)
+          = sortable_item(sort_title_recently_updated, page_filter_path(sort: sort_value_recently_updated, label: true), sort_title)
+          = sortable_item(sort_title_milestone,        page_filter_path(sort: sort_value_milestone,        label: true), sort_title)
+          = sortable_item(sort_title_due_date,         page_filter_path(sort: sort_value_due_date,         label: true), sort_title) if viewing_issues
+          = sortable_item(sort_title_popularity,       page_filter_path(sort: sort_value_popularity,       label: true), sort_title)
+          = sortable_item(sort_title_label_priority,   page_filter_path(sort: sort_value_label_priority,   label: true), sort_title)
+    = issuable_sort_direction_button(sort_value)