feat: Add sorting by exclusive labels (issue priority) (#33206)

Fix #2616

This PR adds a new sort option for exclusive labels.

For exclusive labels, a new property is exposed called "order", while in
the UI options are populated automatically in the `Sort` column (see
screenshot below) for each exclusive label scope.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/repo.css

@@ -1604,6 +1604,12 @@ td .commit-summary {
   margin-right: 0;
 }
 
+.ui.label.scope-middle {
+  border-radius: 0;
+  margin-left: 0;
+  margin-right: 0;
+}
+
 .ui.label.scope-right {
   border-bottom-left-radius: 0;
   border-top-left-radius: 0;