account_key mismatch after changing passwords

[nylen]

Commit dc2b500 removed the code to update the account_key when a user changes their password. However, User#authenticated_by is still called on each login. This changes the account_key in the user table but not in the accounts table, which makes wesabe unable to find any accounts or transactions. Should be reproducible by changing password, logging out, and logging back in.

[eventualbuddha]

Fix that changing the password would disconnect accounts. Closed by 361d28a.