Create VerifierSessionRequired middleware that expects a verifier to exist in session

[thekaveman]

Background

Once the session tracks the selected verifier in #321, we can make use of that infrastructure to put guards on certain view functions that require a verifier to be selected. The first step is to create a new middleware class that enforces the requirement.

This is similar to how the AgencySessionRequired and EligibileSessionRequired middleware are used.

Tasks

• Create a new middleware class like VerifierSessionRequired inheriting from MiddlewareMixin, see the other *SessionRequired as examples
• In process_request(), check session.verifier() for the request.
  • If None, raise an error to stop the request
  • Otherwise return None to allow the request to continue
• Apply this middleware to the following views to enforce that a verifier is needed:
  • eligibility:index
  • eligibility:confirm