Format datetime of subscriptions on admin UI (#4078)

app/views/admin/subscriptions/_subscription.html.haml

@@ -7,10 +7,12 @@
     - if subscription.confirmed?
       %i.fa.fa-check
   %td{ style: "color: #{subscription.expired? ? 'red' : 'inherit'};" }
+    %time.time-ago{ datetime: subscription.expires_at.iso8601, title: l(subscription.expires_at) }
       = precede subscription.expired? ? '-' : '' do
         = time_ago_in_words(subscription.expires_at)
   %td
     - if subscription.last_successful_delivery_at?
+      %time.formatted{ datetime: subscription.last_successful_delivery_at.iso8601, title: l(subscription.last_successful_delivery_at) }
         = l subscription.last_successful_delivery_at
     - else
       %i.fa.fa-times