related tags: fix deprecation warning in order("count(*) DESC").

DEPRECATION WARNING: Dangerous query method (method whose arguments are
used as raw SQL) called with non-attribute argument(s): "count(*) DESC".
Non-attribute arguments will be disallowed in Rails 6.0. This method
should not be called with user-provided values, such as request
parameters or model attributes. Known-safe values can be passed by
wrapping them in Arel.sql(). (called from block in calculate_from_sample
at /home/admin/src/danbooru/app/logical/related_tag_calculator.rb:66)

app/logical/related_tag_calculator.rb

@@ -63,7 +63,7 @@ class RelatedTagCalculator
         posts_with_tags = posts_with_tags.joins("JOIN tags ON tags.name = tag").where("tags.category" => category_constraint)
       end
 
-      counts = posts_with_tags.order("count(*) DESC").limit(max_results).group("tag").count
+      counts = posts_with_tags.order("count_all DESC").limit(max_results).group("tag").count(:all)
       counts
     end
   end