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>

templates/repo/issue/labels/label_list.tmpl

@@ -50,6 +50,7 @@
 							data-label-id="{{.ID}}" data-label-name="{{.Name}}" data-label-color="{{.Color}}"
 							data-label-exclusive="{{.Exclusive}}" data-label-is-archived="{{gt .ArchivedUnix 0}}"
 							data-label-num-issues="{{.NumIssues}}" data-label-description="{{.Description}}"
+							data-label-exclusive-order="{{.ExclusiveOrder}}"
 						>{{svg "octicon-pencil"}} {{ctx.Locale.Tr "repo.issues.label_edit"}}</a>
 						<a class="link-action" href="#" data-url="{{$.Link}}/delete?id={{.ID}}"
 							data-modal-confirm-header="{{ctx.Locale.Tr "repo.issues.label_deletion"}}"