split from #8565 (which was itself spun off from #8516)

You should be able to add new add-on owners/developers via the addon api

Complexity here is

• not actually creating AddonUser, but creating AddonUserPendingConfirmation.
• we need to expose any AddonUserPendingConfirmation somewhere. It would seem like the ideal place is the same /authors/ sub-endpoint but the models themselves are different and not (easily) exposed with a single queryset (there is django magic but probably complete overkill)
• how do users confirm the pending confirmation?