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.
Update README with simplified running instructions
Rename User#create! to User#upsert!

The `User#create!` method was actually creating or updating, rather than
purely creating, and thus calling it `upsert!` is much closer to its true function.
Merge pull request #205 from alphagov/simplify-gds-sso-user-mocking

Simplify gds sso user mocking