Notifications: exclude system users (TM, sync, GT) from recipients

[ayshushus]

Fix #3954

System users (pontoon-sync, pontoon-gt, pontoon-tm) have translation/review activity attributed. Have UserProfile.system_user=True but no callsite check.

• Add profile__system_user=False filter to every notify.send() recipient queryset:
  • sync.core.notify_users (new strings)
  • messaging.management.commands.send_deadline_notifications
  • messaging.management.commands.send_review_notifications (filter on user__profile__system_user)
  • messaging.management.commands.send_suggestion_notifications
  • translations.views.create_translation (new-contributor notification to locale managers)
  • base.views._send_add_comment_notifications and _send_pin_comment_notifications
• Early-return in messaging.notifications.send_badge_notification when user.profile.system_user.
• Filter messaging.views.get_recipients return queryset.

Add tests (@functionzz) in pontoon/messaging/tests/test_send_notifications.py covering all four commands, badge notification; new pontoon/sync/tests/test_notify_users.py covering sync.core.notify_users.

[flodolo]

Passing by thought, since I'm touching some of this code for another issue.

Instead of spreading the filter, have you considered using a helper function in pontoon/messaging/notifications.py to filter+send? e.g. something like

def send_notification(sender, recipient, **kwargs):
    if recipient.profile.system_user:
        return
    notify.send(sender, recipient=recipient, **kwargs)

I think we're filtering out system users before sending notifications in 4/5 places, and I don't see a scenario where we want to notify system users.