Decouple nip 29 roles and permissions

[staab]

Supersedes #2316

This design is mostly backwards compatible (the only breaking change is it swaps out meaningless role strings with meaningful kind strings in the members event). It follows how NIP 43's roles work by decoupling permissions from roles. This way, roles can be cosmetic or meaningful depending on the relay's policy, and any member can be assigned a role. Roles have nothing to do with permissions, which are directly assigned to users (although relays may choose to assign permissions in response to a role assignment). Permissions are group management kind numbers, advertised on the group admins list.