Relinking when Beet loosing the identityhash

Ideally, the developer must only call `beet.get` and `beet-js` recovers by itself unless a severe error occurs.

Assume that Beet and some app have linked, now user re-installs Beet and visits the site again. Site wants to authenticate using the existing link, but Beet does not know it.

Since the identityhash is deterministic, Beet calculates the idendityhash for all imported accounts that match the requested connection details. Two cases:
a) there is match with the client identityhash, user sees a popup in Beet, possibly with a refined message to notify the user of this anomaly, and connection is re-linked. Nothing changes for the developer on the client side
b) there is no match with the client idendityhash. Beet sends an error to the client, and the client has two choices: either destroy the local identityhash and re-link (can happen automatically), or notify the user there and give him a chance to import the correct keys in Beet