Review/remove broken notifications code

[asmecher]

It appears that the notification system used to support notifications with user_id=0, which IIRC used to apply to anyone active on the submission. These notifications do not appear to work anymore (see e.g. NOTIFICATION_TYPE_EDITOR_ASSIGNMENT_... constants).

Some investigation is needed to determine when/why they stopped working, and from there, whether it's worth fixing them -- or whether the whole infrastructure should be torn out. (This would be a good opportunity to reduce complexity and remove that code, since my sense is that it's not badly missed.)

[NateWr]

@asmecher want to keep this one open?

[asmecher]

No, I'll close this, thanks.