Remove User::addEntitlement()

Summary:

• The method is used nowhere and looks quite useless
• Re-activate and fix some stripe tests, fix phpstan errors

Test Plan: ./phpunit

Reviewers: #hkccp_developers, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D2032