Using the API to create a user on an SSO only system

Creating staged users using the API is now possible :deciduous_tree:

https://github.com/discourse/discourse/commit/5bfc9cf69ede127460bc14d1c7f109841d4e8dee
 
https://github.com/discourse/discourse_api/commit/8cfff42d2853c54b7140b2a1fab211ef9cbcbf3b