[feature] Add warning for email verification status and allow to resend verification email

[nemesifier]

If a user is accessing the admin and has email notifications enabled, we should check whether the user has its email verified.
If the email is not verified, we can show a warning message using the django message framework, something like:

Email notifications are enabled for your account, but since your email address has not been verified, email sending is currently disabled. Please verify your email address to enable email notifications.

We should link to a view that triggers the resending of the verification email.

We could use the user_logged_in signal to trigger this message, we should execute the receiver function only if the user is a staff user.