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It is especially useful if there are outdated labels that should no longer be used without deleting the label entirely.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 The time when a label has been archived will be stored in the DB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](https://github.com/go-gitea/gitea/assets/80308335/208f95cd-42e4-4ed7-9a1f-cd2050a645d4)

![image](https://github.com/go-gitea/gitea/assets/80308335/746428e0-40bb-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>

models/fixtures/label.yml

@@ -7,6 +7,7 @@
   exclusive: false
   num_issues: 2
   num_closed_issues: 0
+  archived_unix: 0
 
 -
   id: 2
@@ -17,6 +18,7 @@
   exclusive: false
   num_issues: 1
   num_closed_issues: 1
+  archived_unix: 0
 
 -
   id: 3
@@ -27,6 +29,7 @@
   exclusive: false
   num_issues: 0
   num_closed_issues: 0
+  archived_unix: 0
 
 -
   id: 4
@@ -37,6 +40,7 @@
   exclusive: false
   num_issues: 1
   num_closed_issues: 0
+  archived_unix: 0
 
 -
   id: 5
@@ -47,6 +51,7 @@
   exclusive: false
   num_issues: 0
   num_closed_issues: 0
+  archived_unix: 0
 
 -
   id: 6
@@ -57,6 +62,7 @@
   exclusive: false
   num_issues: 0
   num_closed_issues: 0
+  archived_unix: 0
 
 -
   id: 7
@@ -67,6 +73,7 @@
   exclusive: true
   num_issues: 0
   num_closed_issues: 0
+  archived_unix: 0
 
 -
   id: 8
@@ -77,6 +84,7 @@
   exclusive: true
   num_issues: 0
   num_closed_issues: 0
+  archived_unix: 0
 
 -
   id: 9
@@ -87,3 +95,4 @@
   exclusive: true
   num_issues: 0
   num_closed_issues: 0
+  archived_unix: 0