Refactor the redirect_from_idp method
Refactor the redirect_from_idp method

This commit does:
* fixes this issue: we were not checking that the request params were not nil before starting the request verification (that was causing the `valid_request?` method to fail)
* prepares the ground for adding logging, mainly by restructuring the `redirect_from_idp` method to be a "series of filters", which will be able to easily trigger a specific log message if something goes wrong.

alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend