Function prefill_from_basis 

pub async fn prefill_from_basis(
    pool: &PgPool,
    actor: &Actor,
    revision_id: Uuid,
) -> AppResult<BasisPrefill>

The values the form New simulation based on … is prefilled with. A basis the actor may not read answers like a missing one (404), so the caller can fall back to the plain form without echoing anything.