Increase the expiry duration of the verification link to 2w+1d

Unverified user accounts will be deleted 2 weeks after registration by a chron-job.

• Increase the expiry-duration of the verification link to 2w+1d
• In the e-mail, write that the link will be valid for 2 weeks and the account will be deleted after that duration
• If the user clicks the still-valid link but the account is already deleted, the user will be presented a message that the account has been deleted and the user is supposed to reregister