alert people by email if they are signed up for an existing alert

If someone not logged in, or someone with a different email address
attempts to sign up for an existing alert do not display the already
signed up error on the site but instead send an email to the relevant
email address.

Stops people working out if a user is signed up to an alert by
attempting to sign them up.
If the alert already exists pretend all is well but send an email

Rather than an error which implies that the alert already exists.
Merge branch 'alert_privacy_fix'