Support default Pundit policy class.

There are situations where to have default Pundit policy defined which will handle all cases here dedicated policy could not be found. Some of those cases I described in issue #3319. This PR introduces new ActiveAdmin setting `pundit_default_policy ` which handles authorization when Pundit can't find dedicated Policy class for resource.
Instead of being put in `application.rb`, this should be at the top of `pundit_adapter.rb` similar to [`cancan_adapter.rb`](https://github.com/activeadmin/activeadmin/blob/be5ed27d84dd71ee367cebc6c7bb2124a17d8e63/lib/active_admin/cancan_adapter.rb#L8)