Model refactor: EligibilityVerifier.start_template default value

[thekaveman]

In #1968 we introduced a new model field to hold a template with a default value that can be overridden if needed. The default was stored directly in the model/database rather than separated as a string in the views.

We decided that this is the best approach for these types of cases, so we need to refactor the existing EligibilityVerifier.start_template to conform to this approach.

Acceptance Criteria

• EligibilityVerifier.start_template stores its default value on the model
• eligibility.views.TEMPLATE_START is removed
• tests are updated accordingly

Additional context

The same approach is being taken in #2053

This could be done as part of #2238