FIX: do not use "max_tags_in_filter_list" setting for showing PM tags (#11146)

Ensure we do not respect max_tags_in_filter_list when showing the list of PM tags.

This filter is used on a full page view and there is not point limiting it to a small number.

The expectation is that PM tags are very rarely used, so a hard limit of 1000 should be safe for now.

app/models/tag.rb

@@ -120,9 +120,8 @@ class Tag < ActiveRecord::Base
     tag_names_with_counts.map { |row| row.tag_name }
   end
 
-  def self.pm_tags(limit_arg: nil, guardian: nil, allowed_user: nil)
+  def self.pm_tags(limit: 1000, guardian: nil, allowed_user: nil)
     return [] if allowed_user.blank? || !(guardian || Guardian.new).can_tag_pms?
-    limit = limit_arg || SiteSetting.max_tags_in_filter_list
     user_id = allowed_user.id
 
     DB.query_hash(<<~SQL).map!(&:symbolize_keys!)
@@ -143,6 +142,7 @@ class Tag < ActiveRecord::Base
                                AND gu.group_id = tg.group_id
        )
        GROUP BY tags.name
+       ORDER BY count DESC
        LIMIT #{limit}
     SQL
   end