FIX: Cache failed onebox URL request server-side (#8421)

We already cache failed onebox URL requests client-side, we now want to cache this on the server-side for extra protection. failed onebox previews will be cached for 1 hour, and any more requests for that URL will fail with a 404 status. Forcing a rebake via the Rebake HTML action will delete the failed URL cache (like how the oneboxer preview cache is deleted).

app/controllers/onebox_controller.rb

@@ -19,6 +19,8 @@ class OneboxController < ApplicationController
     invalidate = params[:refresh] == 'true'
     url = params[:url]
 
+    return render(body: nil, status: 404) if Oneboxer.recently_failed?(url)
+
     hijack do
       Oneboxer.preview_onebox!(user_id)
 
@@ -34,6 +36,7 @@ class OneboxController < ApplicationController
       Oneboxer.onebox_previewed!(user_id)
 
       if preview.blank?
+        Oneboxer.cache_failed!(url)
         render body: nil, status: 404
       else
         render plain: preview