Break dependency between Scalar Types and eligibility conditions

[greg-hyde]

Describe the bug
Right now, the eligibility conditions are tied very tightly to the set of scalars defined by Scalar.java. In one recent instance, we added new scalar types for date questions that were UI concepts only and were not stored in the database, and found that the scalar values appeared in the eligibility conditions dropdown. We probably want to use an allowlist on the EligibilityPredicateView to ensure we only allow adding eligibility conditions for scalars we explicitly want to support conditions on