Store email aliases in LDAP

Summary:
Use 'alias' attribute in LDAP for user aliases.

Fixes aliases update, added proper tests.

Test Plan: ./phpunit

Reviewers: #hkccp_developers, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: vanmeeuwen

Differential Revision: https://git.kolab.org/D1174