Allow to archive labels (#26478)

## Archived labels 

This adds the structure to allow for archived labels.
Archived labels are, just like closed milestones or projects, a medium to hide information without deleting it.
It is especially useful if there are outdated labels that should no longer be used without deleting the label entirely.

## Changes

1. UI and API have been equipped with the support to mark a label as archived
2. The time when a label has been archived will be stored in the DB

## Outsourced for the future

There's no special handling for archived labels at the moment.
This will be done in the future.

## Screenshots

![image](208f95cd-42e4-4ed7-9a1f-cd2050a645d4)

![image](746428e0-40bb-45b3-b992-85602feb371d)

Part of https://github.com/go-gitea/gitea/issues/25237

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/issue_label_test.go

@@ -97,9 +97,10 @@ func TestUpdateLabel(t *testing.T) {
 	test.LoadUser(t, ctx, 2)
 	test.LoadRepo(t, ctx, 1)
 	web.SetForm(ctx, &forms.CreateLabelForm{
-		ID:    2,
-		Title: "newnameforlabel",
-		Color: "#abcdef",
+		ID:         2,
+		Title:      "newnameforlabel",
+		Color:      "#abcdef",
+		IsArchived: true,
 	})
 	UpdateLabel(ctx)
 	assert.EqualValues(t, http.StatusSeeOther, ctx.Resp.Status())