CLI tool to convert server-side account to client-side account

[tmpfs]

Servers store head-only vaults but clients need the secrets in the vaults so we should create a tool to convert the server-side accounts into client-side accounts.

This could be used to recover a client-side account from a server-side account if necessary.

Note that recovering the device vault is impossible as only the device events (Trust and Revoke) are shared with the server.

See ClientStorage::rebuild_folder_vault().