make identity name optional

Currently identity name is required:

"Name"
name: String! @index

but until the identity is confirmed, its value is set to the id:

name: identity.event.id, // Using the id of the creation event as the name for unconfirmed identities

This can be confusing. In addition, we have to handle the consequences of prune_item_identities_names blockchain root call, which will make names available again and identities lose their name.

Client side, these identities without name will be displayed as "unconfirmed" is name is not set yet and "deleted" if identity or name has been removed from blockchain.