Add a new "start eligibility" view/URL

[thekaveman]

This replaces what the current eligibility:index view is doing, which will transition to displaying the verifier selection form in #322.

Depends on #319, #323

• Rename the instructions_* fields from Update EligiblityVerifier model to support custom text content #319 to start_* to align with the simplification in this task
  • models
  • migrations
  • fixtures
• Update *.po files with a new keys eligibility.pages.start.*, copy the value from eligibility.pages.index.* (for now)
• Create view function named start
  • Add an AgencySessionRequired middleware decorator to this view
  • Add a VerifierSessionRequired middleware decorator to this view
• Create a URL route /eligibility/start to this view
• In the view:
  • Read from session the selected verifier
  • Create a Page viewmodel using a mix of the common keys and the verifier-specific keys for text translations
  • Send the viewmodel to the template with the PageTemplateResponse helper