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It is especially useful if there are outdated labels that should no longer be used without deleting the label entirely.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 The time when a label has been archived will be stored in the DB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>

services/forms/repo_form.go

@@ -569,6 +569,7 @@ type CreateLabelForm struct {
 	ID          int64
 	Title       string `binding:"Required;MaxSize(50)" locale:"repo.issues.label_title"`
 	Exclusive   bool   `form:"exclusive"`
+	IsArchived  bool   `form:"is_archived"`
 	Description string `binding:"MaxSize(200)" locale:"repo.issues.label_description"`
 	Color       string `binding:"Required;MaxSize(7)" locale:"repo.issues.label_color"`
 }