Aliases sharing
Summary: Support assigning the same alias to different users (of the same account)
Test Plan: php artisan migrate:refresh --seed; ./phpunit
Reviewers: #hkccp_developers, vanmeeuwen
Reviewed By: #hkccp_developers, vanmeeuwen
Subscribers: vanmeeuwen, #hkccp_developers
Differential Revision: https://git.kolab.org/D1402