Expire EmailVerifications / Raise 404 on invalid EmailVerification

This pull request solves two problems in the emailverification app:

1. A user is able to create multiple `EmailVerification` objects that are not expired. This is not the expected behavior. Only the most recent verification request should be valid.
2. In the `EmailChangeApproveView` an invalid `EmailVerification` will correctly flash an error message. However, the response is a redirect to a page stating a successful request. The change will raise a `HTTP404` if the request is invalid.