Add feature detection for unmanaged groups

[staab]

Feature detection for unmanaged groups has to exist on two levels:

• On the group level, so that clients can toggle the richer NIP 29 behavior on or off. Feature detection based on NIP 11's supported_nips field isn't sufficient, since a relay implementation might support NIP 29, but the actual relay may not.
• On the relay level, since clients need to be able to know whether users can post events to groups that don't yet exist, or whether they have to send a create-group event instead.

The default (obviously) has to be that unmanaged groups are allowed, so unmanaged group support is opt-out.