[51][83]: add user creation; add generic server error handling

[mi4l]

Issue

Closes #51, #83

This allows us to post to /users with a request resulting in the creation of a new user.

Example request body posted to <site>/users:

{
    "username": "brickmaster281",
    "email": "masonthebuilder@gmail.com",
    "authentication": {
        "basic": {
            "username": "brickmaster281",
            "password": "123456"
        }
    }
}

Results in the creation of a new user:

Via service client:

/review @JacobPernell @mskalandunas