POST /site CreateSite

[ShadowJonathan]

I'm abstracting this out of #52 because it has an extremely rare usecase of being only used when first setting up an instance.

We may pair this with an in-app option to deploy instances at some hosting service, maybe, but until then, this is basically the rarest API call.

It is also a lot to write down, so I'll ignore it for now (for #52) and write the other ones.