Anonymous user creation via API

It would be great if there was a setting to allow anonymous users to be created via POST /api/users. It can be optional in the galaxy config.

It also would be great to have a request param that causes an api key to be returned with the response to save a second request to /api/authentication.

Start by tracing the POST /api/users entry point and how Galaxy configuration settings are defined and read. Then inspect /api/authentication to understand the existing API-key flow. Done means an optional configuration permits anonymous user creation and an optional request parameter returns an API key in the creation response, with behavior covered by tests.

Written by the indexing model from the issue text.