[feature request] Share nPub key with qrcode #79

Please add a way directly on the app to share the nPub key with qrcode, and also a way to catch it to add a new contact.

Also, as it is possible to create public link for groups, it could be good to even generate a qrcode for the invites, and the way to catch them and join to the group.

This to make it easier, faster an so user-friendly to add contacts and join groups during face to face situations.