Modify resources when changing groups

[ThomasKranitsas]

@rootelement what should happen in the following case:

• Copilot creates a challenge with either no groups or with, let's say, Group1
• A few members who have access to Group1 register on the challenge.
• Copilot modifies the challenge and adds Group2

Possible scenarios:

1. All members who don't have access to Group2 will be automatically removed
2. Copilot should not be able to do that
3. Allow the copilot to do that and leave already registered members (resources) to be registered (doesn't sound like a good approach to me)

cc @cwdcwd