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>

templates/swagger/v1_json.tmpl

@@ -20221,6 +20221,11 @@
           "type": "boolean",
           "x-go-name": "Archived"
         },
+        "archived_at": {
+          "type": "string",
+          "format": "date-time",
+          "x-go-name": "ArchivedAt"
+        },
         "avatar_url": {
           "type": "string",
           "x-go-name": "AvatarURL"