posts: reduce string allocations during thumbnail generation.

Further micro-optimize thumbnails to reduce string allocations.

`Post#levelblocked?` gets called once per thumbnail. Before it split the
tag string, which meant one string allocation for each tag on each post.
This added up to thousands of string allocations per pageload.

app/models/post.rb

@@ -8,7 +8,7 @@ class Post < ApplicationRecord
   NOTE_COPY_TAGS = %w[translated partially_translated check_translation translation_request reverse_translation
                       annotated partially_annotated check_annotation annotation_request]
 
-  RESTRICTED_TAGS = Danbooru.config.restricted_tags
+  RESTRICTED_TAGS_REGEX = /(?:^| )(?:#{Danbooru.config.restricted_tags.join("|")})(?:$| )/o
 
   self.ignored_columns = [:pool_string, :fav_string]
 
@@ -1289,7 +1289,8 @@ class Post < ApplicationRecord
   end
 
   def levelblocked?(user = CurrentUser.user)
-    !user.is_gold? && RESTRICTED_TAGS.any? { |tag| tag.in?(tag_array) }
+    #!user.is_gold? && RESTRICTED_TAGS.any? { |tag| has_tag?(tag) }
+    !user.is_gold? && tag_string.match?(RESTRICTED_TAGS_REGEX)
   end
 
   def banblocked?(user = CurrentUser.user)