add og:published_time to opengraph meta tags (#14865)

app/views/statuses/show.html.haml

@@ -12,6 +12,7 @@
   = opengraph 'og:type', 'article'
   = opengraph 'og:title', "#{display_name(@account)} (#{acct(@account)})"
   = opengraph 'og:url', short_account_status_url(@account, @status)
+  = opengraph 'og:published_time', @status.created_at.iso8601
 
   = render 'og_description', activity: @status
   = render 'og_image', activity: @status, account: @account