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It is especially useful if there are outdated labels that should no longer be used without deleting the label entirely.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 The time when a label has been archived will be stored in the DB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

modules/structs/issue_label.go

@@ -11,6 +11,8 @@ type Label struct {
 	Name string `json:"name"`
 	// example: false
 	Exclusive bool `json:"exclusive"`
+	// example: false
+	IsArchived bool `json:"is_archived"`
 	// example: 00aabb
 	Color       string `json:"color"`
 	Description string `json:"description"`
@@ -27,6 +29,8 @@ type CreateLabelOption struct {
 	// example: #00aabb
 	Color       string `json:"color" binding:"Required"`
 	Description string `json:"description"`
+	// example: false
+	IsArchived bool `json:"is_archived"`
 }
 
 // EditLabelOption options for editing a label
@@ -37,6 +41,8 @@ type EditLabelOption struct {
 	// example: #00aabb
 	Color       *string `json:"color"`
 	Description *string `json:"description"`
+	// example: false
+	IsArchived *bool `json:"is_archived"`
 }
 
 // IssueLabelsOption a collection of labels