Setup a system for creating links that can be used in emails for registering reasons for agreement ending

[fellmirr]

When donors end an agreement, we will allow them to give a reason for their ending the agreement. We want to add a little bit delay before sending the agreement ended email then, and depending on wether they provided a reason, we want to include an option for them to provide a reason in the email we send. We can generate unique links in a table in the backend that uniquely connect a donor and a reason registered and add those in the email for one-click registration.