feat(server): create a repository over a private admin API #84

A repository is created with origin repo create on the server's own machine. A host needs to create one from its own service, when a person asks it to.

Nothing is created implicitly: a request that names a repository the server hasn't created is refused, even if the registry (#53) lists it. The host's service calls an explicit create, with the repository's ID and its first admin.

The call is on a private listener, apart from the public API, so it never crosses the public internet and only the host's own services reach it. Other calls between a host's services and the server belong there too.

Plan: sourceorigin-dev/sourceorigin#527, docs/plans/platform/.