Fix #5001: Media assets not searchable through upload records.

Fix this:

  https://danbooru.donmai.us/uploads.json?search[media_assets][md5]=b83daa7f1ae7e4127b1befd32f71ba10

failing with an ActiveRecord::StatementInvalid error.

The bug was that for a `has_many through: ...` association, like
`has_many :media_assets, through: :upload_media_assets`, we weren't
joining on the associated table properly so we ended up generating
invalid SQL.

app/logical/concerns/searchable.rb

@@ -436,7 +436,7 @@ module Searchable
     end
 
     if parameter_hash?(params[attr])
-      relation = relation.where(attr => model.visible(current_user).search(params[attr]).reorder(nil))
+      relation = relation.includes(attr).references(attr).where(attr => model.visible(current_user).search(params[attr]).reorder(nil))
     end
 
     relation