FIX: use allowlist and blocklist terminology (#10209)

This is a PR of the renaming whitelist to allowlist and blacklist to the blocklist.

app/models/post_analyzer.rb

@@ -52,7 +52,7 @@ class PostAnalyzer
 
     cooked_stripped.css("img").reject do |t|
       if dom_class = t["class"]
-        (Post.white_listed_image_classes & dom_class.split).count > 0
+        (Post.allowed_image_classes & dom_class.split).count > 0
       end
     end.count
   end