organization_list all_fields misses out extras and users

(Description copied from issue https://github.com/ckan/ckan/issues/1768 )

organization_list misses out the extra fields of organizations. The extras store pretty core information about the organization, so it is a shame. The only way to get the extra fields is to call organization_show for each one, and it's annoyed me a few times now.

While we're at it we might add in the users attached to the organization, plus a few other little fields too.

It's been noted in #759 that that the organization dict returned is different between organization_list (using all_fields) and organization_show. It was decided that the packages property was not going to be changed, for backwards compatibility. But the extras and users are the other remaining properties and they seem fair game to add to organization_list.

Users were to slow to get, but I have made extras, tags & groups optional.