Move newsletter User scope outside UserSegments

Why:

UserSegments are not only used for Newsletters or Email downloads, but
also for internal Global Notifications. We don't want to have that scope
hardcoded inside UserSegments as users that have opted-out from the
newsletter should still be recipients of global notifications.

How:

Removing the scope from the UserSegments `all_users` method that acts as
base for all the other segments. Including that `newsletter` scope only
on the places that is relevant:
* When listing recipients for a newsletter
* When downloading a listing emails that can be newsletter recipients

Also updated relevant tests

spec/lib/user_segments_spec.rb

@@ -6,15 +6,11 @@ describe UserSegments do
   let(:user3) { create(:user) }
 
   describe "#all_users" do
-    it "returns all active users with newsletter enabled" do
-      active_user1 = create(:user, newsletter: true)
-      active_user2 = create(:user, newsletter: true)
-      active_user3 = create(:user, newsletter: false)
+    it "returns all active users enabled" do
+      active_user = create(:user)
       erased_user  = create(:user, erased_at: Time.current)
 
-      expect(described_class.all_users).to include active_user1
-      expect(described_class.all_users).to include active_user2
-      expect(described_class.all_users).not_to include active_user3
+      expect(described_class.all_users).to include active_user
       expect(described_class.all_users).not_to include erased_user
     end
   end