Auto-update the system status in admin dashboard (#29163)

- Refactor the system status list into its own template
- Change the backend to return only the system status if htmx initiated
the request
- `hx-get="{{$.Link}}/system_status`: reuse the backend handler
- `hx-swap="innerHTML"`: replace the `<div>`'s innerHTML (essentially
the new template)
- `hx-trigger="every 5s"`: call every 5 seconds
- `hx-indicator=".divider"`: the `is-loading` class shouldn't be added
to the div during the request, so set it on an element it has no effect
on
- Render "Since Last GC Time" with `<relative-time>`, so we send a
timestamp

# Auto-update in action GIF

![action](https://github.com/go-gitea/gitea/assets/20454870/c6e1f220-f0fb-4460-ac3b-59f315e30e29)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>

routers/web/web.go

@@ -677,6 +677,7 @@ func registerRoutes(m *web.Route) {
 	// ***** START: Admin *****
 	m.Group("/admin", func() {
 		m.Get("", admin.Dashboard)
+		m.Get("/system_status", admin.SystemStatus)
 		m.Post("", web.Bind(forms.AdminDashboardForm{}), admin.DashboardPost)
 
 		m.Get("/self_check", admin.SelfCheck)