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.

Ordinarily that'd be the case, but in this app the `User` model uses hand-rolled redis-backed persistence, for which the [`create!` method](https://github.com/alphagov/support/blob/master/app/models/user.rb#L50) actually overwrites the existing user.