Django form to capture user's EligibilityVerifier selection

[thekaveman]

Background

Building on #320 and #332, once a TransitAgency has multiple EligibilityVerifier references, the user needs a way to select which one they will use for verification.

Django has the ChoiceField which can use the widget=RadioSelect to render as a list of radio buttons. The field takes a list of choices, each a tuple where the first item is the value and the second is the label:

from django import forms

choices = [("value1", "Label One"), ("value2", "Label Two")]
field = forms.ChoiceField(choices=choices, widget=forms.RadioSelect)

See e.g. https://stackoverflow.com/a/5925240/453168

Tasks

• Create a new form class like EligibilityVerifierSelectionForm in benefits/eligibility/forms.py; form should POST to eligibility:index
• Add a ChoiceField using widget=RadioSelect
• Create an __init__() method accepting an agency parameter
• Seed the ChoiceField.choices from the agency.eligibility_verifiers property, using the above choices structure as an example
• Rework the eligibility:index view to handle both GET and POST
  • for GET: If agency has multiple verifiers, initialize form with the selected agency's verifiers and send to the template; otherwise store the one verifier in session and immediately redirect to eligibility:start
  • for POST: store the selected verifier in session and redirect to eligibility:start

[thekaveman]

FYI @angela-tran I moved a few of these items (creating the new sample data) to #338, so this one is now strictly to implement the form logic.