Consider sharing coordinates for pub keys

[willyfromtheblock]

The group participant setup could share the x-coordinate of a public key (pubkey.x) for the identifier. When adding other participants it could ask for the public key and use the x-coord identifier by default. However, it could also have an "advanced" dropdown to add a custom identifier or seed (name) if necessary to be compatible with other potential applications.

I could adapt the group config YAML to allow for a list of public keys in place of a identifier:key mapping. However, I think it isn't worth doing that right now. It's possible to have an identifier in the mapping that is the same as the public key x-coord.

Originally posted by @MatthewLM in #286

[MatthewLM]

This is the same issue as #304