Allow inherited users to be selected for notifications.

This uses the new ``oa_core_get_inherited_users_from_space()`` function added in https://github.com/phase2/oa_core/pull/76 to get the list of users that can be notified, so now inherited users can be selected too.

NOTE: This is totally different than this issue on Drupal.org, which is about making Group notifications also notify inherited users:

https://www.drupal.org/node/2194169

... and NOT about allowing User notifications have inherited users selected, which is what this PR is about. I hope that makes sense. :-)