Keep record of email change history

[ffont]

As a Freesound admin, I want to keep a record of the email addresses assigned to a user account and when/who changed them so if needed (e.g., in case of account theft) I can have that information.

This could be done by creating OldEmail objects in a similar way in which we create OldUsername. We should store the date, the old email, the new email, and the username of the user who changed it (either the actual user or an admin). Also we should make sure these objects are removed when a user is anonymized or deleted.

[alastair]

This should happen both when a user changes their email address through the website, and when an admin changes it.