Usergroups can be linked to an LDAP auth_source. This allows usergroups
to be on sync with its equivalent LDAP usergroup.

Relation between auth_sources and usergroups

Tests, validations, and actual LDAP queries

Rake task to periodically refresh LDAP user groups

Adding LDAP users is done through User.where, not iterating through all possible users

Migration for groups/user OU, recursive usergroup search

Non recursive LDAP rake task

Check against LDAP aliases, or other empty results