[feature/auth-refactor] Make LDAP consistent with interface

Makes the provider_ldap consistent with the provider_interface
except for the acp() method which has not yet been finalized.
Renames phpbb_auth_provider_ldap::user_filter to
phpbb_auth_provider_ldap::ldap_user_filter to maintain the original
name of the function from auth_ldap.

PHPBB3-9734