Create web users, create projects, and assign users to projects

I wrote a [provisioning script](https://gist.github.com/yanokwa/480d6d5839bd73a8956cbf88ea83fdac) to create web users, projects, and assign users to projects. I had to use the REST API and I think it'd be nice to have pyODK have. And to be explicit, I don't think there should be a method that does this in one go. I'm arguing for methods to do each of these individually.

I haven't thought at all about what they pyODK methods would be called and what they'd be associated with. Some are pretty obvious (e.g.,`projects.create(project_name, project_description)`) but where users and assignment are require some thinking.

Start with the linked provisioning script and the REST API calls it uses, then determine suitable pyODK entry points for separate project, user, and assignment operations. Done means the requested individual methods are designed and implemented without combining them into one operation.

Written by the indexing model from the issue text.