saved searches: fix exploit allowing flaggers to be determined.

Fix an exploit that let you determine the flagger of a post using
`flagger:<username>` saved searches. Saved searches were performed as
DanbooruBot, but since DanbooruBot is a moderator, it let unprivileged
users do `flagger:<username>` searches. Saved searches were done as a
moderator to avoid tag limits, but this is no longer necessary since the
last PostQueryBuilder refactor.

fred get out

app/models/saved_search.rb

@@ -130,7 +130,7 @@ class SavedSearch < ApplicationRecord
         return if redis.exists?(redis_key)
 
         post_ids = Post.with_timeout(timeout, [], query: query) do
-          Post.system_tag_match(query).limit(QUERY_LIMIT).pluck(:id)
+          Post.anon_tag_match(query).limit(QUERY_LIMIT).pluck(:id)
         end
 
         if post_ids.present?