FIX: Make sure max_oneboxes_per_post is enforced (#16215)

PostAnalyzer and CookedPostProcessor both replace URLs with oneboxes.
PostAnalyzer did not use the max_oneboxes_per_post site and setting and
CookedPostProcessor replaced at most max_oneboxes_per_post URLs ignoring
the oneboxes that were replaced already by PostAnalyzer.

lib/cooked_processor_mixin.rb

@@ -3,7 +3,7 @@
 module CookedProcessorMixin
 
   def post_process_oneboxes
-    limit = SiteSetting.max_oneboxes_per_post
+    limit = SiteSetting.max_oneboxes_per_post - @doc.css("aside.onebox, a.inline-onebox").size
     oneboxes = {}
     inlineOneboxes = {}