Use node id as external id

[Guillembonet]

We could use node id as external id when creating conversations and then users would have their full history even when migrating a node. That will require some changes in the intercom settings (disabling identity verification) or a new endpoint that allows users to compute their user hash if they send a signed message from node with the requested identity.
This endpoint would need to always be called on node UI load if there is no cookie stored to persist the user_id and user_hash.

The latter would be preferred as otherwise, users would be able to impersonate any node runner.