Link prefetching brings confusion into the account activation process

[anthonygego]

For now we store an activation hash which is removed from the User document in database once the account is activated. Therefore, the activation link is not valid anymore once the user is activated.

However some browsers in combination with online mail clients actually perform link prefetching. In some cases, a recently registered user faces an "Invalid token" message instead of "Your account is already/has been activated".

A possible solution is to keep the activation hash somewhere in DB to inform the user the account has already been activated.