Preserve label filters when sorting

Closes #20982

https://gitlab.com/gitlab-org/gitlab-ce/issues/20982

CHANGELOG

@@ -14,6 +14,7 @@ v 8.13.0 (unreleased)
   - Close open merge request without source project (Katarzyna Kobierska Ula Budziszewska)
   - Use a ConnectionPool for Rails.cache on Sidekiq servers
   - Replace `alias_method_chain` with `Module#prepend`
+  - Preserve label filters when sorting !6136 (Joseph Frazier)
   - Only update issuable labels if they have been changed
   - Take filters in account in issuable counters. !6496
   - Revoke button in Applications Settings underlines on hover.

app/views/shared/_sort_dropdown.html.haml

@@ -8,26 +8,26 @@
     %b.caret
   %ul.dropdown-menu.dropdown-menu-align-right.dropdown-menu-sort
     %li
-      = link_to page_filter_path(sort: sort_value_priority) do
+      = link_to page_filter_path(sort: sort_value_priority, label: true) do
         = sort_title_priority
-      = link_to page_filter_path(sort: sort_value_recently_created) do
+      = link_to page_filter_path(sort: sort_value_recently_created, label: true) do
         = sort_title_recently_created
-      = link_to page_filter_path(sort: sort_value_oldest_created) do
+      = link_to page_filter_path(sort: sort_value_oldest_created, label: true) do
         = sort_title_oldest_created
-      = link_to page_filter_path(sort: sort_value_recently_updated) do
+      = link_to page_filter_path(sort: sort_value_recently_updated, label: true) do
         = sort_title_recently_updated
-      = link_to page_filter_path(sort: sort_value_oldest_updated) do
+      = link_to page_filter_path(sort: sort_value_oldest_updated, label: true) do
         = sort_title_oldest_updated
-      = link_to page_filter_path(sort: sort_value_milestone_soon) do
+      = link_to page_filter_path(sort: sort_value_milestone_soon, label: true) do
         = sort_title_milestone_soon
-      = link_to page_filter_path(sort: sort_value_milestone_later) do
+      = link_to page_filter_path(sort: sort_value_milestone_later, label: true) do
         = sort_title_milestone_later
       - if controller.controller_name == 'issues' || controller.action_name == 'issues'
-        = link_to page_filter_path(sort: sort_value_due_date_soon) do
+        = link_to page_filter_path(sort: sort_value_due_date_soon, label: true) do
           = sort_title_due_date_soon
-        = link_to page_filter_path(sort: sort_value_due_date_later) do
+        = link_to page_filter_path(sort: sort_value_due_date_later, label: true) do
           = sort_title_due_date_later
-      = link_to page_filter_path(sort: sort_value_upvotes) do
+      = link_to page_filter_path(sort: sort_value_upvotes, label: true) do
         = sort_title_upvotes
-      = link_to page_filter_path(sort: sort_value_downvotes) do
+      = link_to page_filter_path(sort: sort_value_downvotes, label: true) do
         = sort_title_downvotes