Simplify gds sso user mocking

This change sets the `gds-sso` test user automatically in development mode, whenever SSO is mocked out. This means that:

* it's no longer necessary to run a rake task once a day to run this app in development.
* 2 methods from `User` (that were only used in SSO mocking mode) can be dropped.

The README has been updated to reflect these changes.

after a real-world discussion, changed the method name to `upsert!` here: 0ff86be