New feature "transfer to new account"

[flash1293]

In order to save data over from an account which may be compromised.

• client: Add new menu point in sidebar: "Logout all other devices (you will need to redistribute the sync link again)"
• client: On click dispatch an appropriate action which performs a POST /token for a new account. In the payload add object: transferFromAccount: { username, password }. Don't purge the state, but just replace username/password/token.
• server: Check whether username/password in transfer from account are correct and copy stream data into new key if this is the case. Then delete the old stream and the old password hash (to free disk space)