Add checkbox to accept code of conduct when updating profile

[athomschke]

• Links to CoC in new tab
• Needs to be checked in order to submmit the changes
• Is not persisted - needs to be accepted each time the profile is updated

#1116

[athomschke]

[tyranja]

@athomschke

@zaziemo and I gave this feature some thoughts yesterday and we would rather do a different kind of approach here. Because we are afraid some users will not know what to do here and get stuck.

1. We save the confirmation in the DB and add some validation in the model with a good error message, so the user know exactly why saving failed
2. We used the bootstrap validation-> https://getbootstrap.com/docs/5.1/forms/validation/. Here we also get a understandable error message

Let's talk about that next meeting 💕

[athomschke]

We just talked - the agreement is that we store the flag per-profile, so that Speakerinnen, once this is merged, need to accept the CoC one time. Ideally, a Speakerin also cannot uncheck this setting once it has been accepted once.