Class-based views: Eligibility index

View function: https://github.com/cal-itp/benefits/blob/main/benefits/eligibility/views.py#L22

## Acceptance Criteria



- [x] Mixin class enforces `AgencySessionRequired`
- [x] Mixin class enforces `RecaptchaEnabled`
- [x] `get()` and `post()` methods

## Additional context