Support for old locks

[bastianallgeier]

Summary of changes

• New Lock::legacy() method to return a lock instance based on an old .lock file
• New Lock::legacyFile() method to return an absolute path to an old lock file based on the model
• The PreviewView now shows a custom message for old lock states because it cannot display the current changes from other users.
• New cleanup method in the Changes controller to remove old lock files as soon as new changes are stored or discarded