Further improvements to suspicious user handling

[doobry-systemli]

• OneToOne relation between voucher and user.invitedVoucher was broken, and so was the app:voucher:unlink command.
• Check for invited by ROLE_SUSPICIOUS when assigning the role. Instead of running this check in the cronjob when unlinking vouchers, we want to know this immediately when setting a user as suspicious. (Fixes: #215)
• Don't accept invite codes of suspicious users on registration.

[sonarcloud[bot]]

Quality Gate failed

Failed conditions
69.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud