enable ldap filters for user logins

This pull request will enable filter queries against ldap which is particularly useful when you have a big ldap directory and want to give access only to a subset of your users to your gitlab instance.

Please note that this pull request was migrated from omniauth-ldap project pull request 22 which was developed and authored by sdeframond and can be accessed at this link: https://github.com/intridea/omniauth-ldap/pull/22