Allow new users to be created in specific directories via the REST API

According to Crowd REST APIs, POST /user can be used to create a new user, and that user will be created in the first writeable directory in directory order for the application making the request.

As an improvement, we could allow the ability to specify a directory when adding users. This could make the process more transparent and give the administrator more control by taking out the element of chance, in the event the administrator is unaware of the directories mapped to an application and the order they're in.