Interface to add/remove coordinators and organizers

[slaporte]

@yarl and I discussed creating an interface to add or remove coordinators and organizers. I added an endpoint (/v1/admin/users) that will show all the coordinators/organizers/maintainers in the system. The interface is currently only visible to organizers and maintainers. Note, we can't add or remove maintainers via this interface -- they are hard-coded into the system. If you want, we can treat them as permanent organizers.

In the future, we may want to scope this interface by series (a group of campaigns, like Wiki Loves Monuments or Wiki Loves Earth).