Track EligibilityVerifier in user session

[thekaveman]

Background

As part of #317 the application will need maintain the user's selected EligibilityVerifier.

We can do this using the existing session framework which keeps everything local to the user in secure cookies.

Tasks

• Add a new key _VERIFIER = "verifier" for use elsewhere
• Create a new function verifier() that uses the key to get the EligibilityVerifier instance corresponding to the saved value from the session; see agency() and eligibility() as examples
• Update the update() function to accept a new kwarg verifier=None; if not None and it is a valid EligibilityVerifier instance, store the id in the request's session using the key
• Update the reset() function to also clear the verifier
• Update the context_dict() function to add the new key/value - this shows up in the debug bar
• Store the agency's first verifier in session (for now) when an agency is configured
• Retrieve the verifier from session to give to the API client