Alternative to https://github.com/thoughtbot/administrate/pull/1264. Fixes https://github.com/thoughtbot/administrate/issues/1179.
Currently, Administrate::Search::Query makes the assumption that an association's table name will have the same name as the association attribute, ignoring a :class_name option that signals otherwise.
This change uses the :class_name option when given.
Changing the example app also provides a way to test this as part of the existing feature specs.