Display when a repo was archived (#22664)

This adds the date a repo is archived to Gitea and shows it in the UI
and API. A feature, that GitHub has been [introduced
recently](https://github.blog/changelog/2022-11-23-repository-archive-date-now-shown-in-ui/).

I currently don't know how to correctly deal with the Date in the
template, as different languages have different ways of writing a date.


![grafik](https://user-images.githubusercontent.com/15185051/234315187-7db5763e-d96e-4080-b894-9be178bfb6e1.png)

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/structs/repo.go

@@ -80,6 +80,7 @@ type Repository struct {
 	Created time.Time `json:"created_at"`
 	// swagger:strfmt date-time
 	Updated                       time.Time        `json:"updated_at"`
+	ArchivedAt                    time.Time        `json:"archived_at"`
 	Permissions                   *Permission      `json:"permissions,omitempty"`
 	HasIssues                     bool             `json:"has_issues"`
 	InternalTracker               *InternalTracker `json:"internal_tracker,omitempty"`