Ignore "system" users (TM, sync, GT) in notifications

My local install only has 1 user. While testing notifications, I noticed it would send it to 5 users.

In Django admin there is one user that is completely empty (can't even click it). Why? Is it created at the start?

Then there are 3 "system" users (`pontoon-gt`, `pontoon-sync`, `pontoon-tm`). We shouldn't send notifications to these.