[FIX] mail: use explicit 'active' domain for user count in ping

Before the migration to the new API of this code, the context was
not propagated to the update_notification call.

This is now the case, which is unfortunate since the context of the
cron action has an 'active_test'=False key, which causes the ping
to count user differently when the cron is triggered from the form
view.

This commit adds explicit ('active','=',True) domain to ensure we
only count active users when pinging.