LDAP: Read email and quota when mapping user, fixes #7785

Small-Footpring fix for #7785 on stable6. Puts methods to retrieve information for a specific user into LDAP interaction class, so it is available when creating the user reference in owncloud as well as through the user backend. More sophisticated fix for master is in a PR of it's own.

Please review (and if possible test :D) @uka-support @leo-b @PVince81  or others, please.