Group Database backend must not gather user details itself but ask user

backends. This is a port to master from PR #7745

remove OC_GROUP_BACKEND_GET_DISPLAYNAME option for group backends

Conflicts:
	lib/private/group/backend.php

LDAP: getDisplayNamesInGroup is not an option for group backends anymore

Conflicts:
	apps/user_ldap/group_ldap.php
	apps/user_ldap/group_proxy.php

clean up group backends

Conflicts:
	lib/private/group/database.php

remove now unnecessary test

implement getDisplayNames in group manager

adjust user manager tests

test for group manager's displayNamesInGroup

trim must not be used in empty in PHP < 5.5

keep the constant to not provoke PHP warnings

Conflicts:
	lib/private/group/backend.php