Allow notifications to target existing users only

[damithc]

context: the admin can create a notification targeting following categories of users

I'm wondering if it is worthwhile to be able to target existing users only, existing instructors in particular. This can be useful when we want to communicate a change to an existing feature, which does not concern users joining TEAMMATES in the future. In fact, we don't want to bother new users with this kind of 'legacy' information at the very start.

How's the implementation and operating cost of such a feature would be?

An alternative is to email all existing users, but that's a bit spamy.