Allow an alias to be added forcefully

Summary: Allow an alias to be added forcefully (alias to existing user that used to be the email address for a previously deleted user).

Test Plan: None

Reviewers: #hkccp_developers, machniak

Reviewed By: #hkccp_developers, machniak

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D1582