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It is especially useful if there are outdated labels that should no longer be used without deleting the label entirely.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 The time when a label has been archived will be stored in the DB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

options/locale/locale_en-US.ini

@@ -1491,6 +1491,8 @@ issues.label_title = Name
 issues.label_description = Description
 issues.label_color = Color
 issues.label_exclusive = Exclusive
+issues.label_archive = Archive Label
+issues.label_archive_tooltip= Archived labels are excluded from the label search when applying labels to an issue. Existing labels on issues remain unaffected, allowing you to retire obsolete labels without losing information.
 issues.label_exclusive_desc = Name the label <code>scope/item</code> to make it mutually exclusive with other <code>scope/</code> labels.
 issues.label_exclusive_warning = Any conflicting scoped labels will be removed when editing the labels of an issue or pull request.
 issues.label_count = %d labels