Allow Karrot instance to make phone number field obligatory

User story: When a new person joins my group, they agree by signing a digital form that their e-mail address and phone number are shared with the other group members “for the sole purpose of organisation”. Currently, people only have to put an e-mail address on Karrot, while the phone number is optional. In some urgent situations however, reaching people by phone is crucial, especially without Internet connection (if you don’t have data on your phone, if you have no smartphone or if there is no 4G/5G network coverage). Such situations include for example late activity dropouts due to accidents or being stuck in traffic.

Complexity: easy, I guess

Outcomes (or what needs to be done to move forward): I think an option should be added to the .env file and exposed via the co-op cloud recipe too.