Function upsert_exercises

async fn upsert_exercises(
    conn: &mut PgConnection,
    page: &Page,
    existing_exercise_ids: &[Uuid],
    exercise_updates: &[CmsPageExercise],
    retain_exercise_ids: bool,
) -> ModelResult<HashMap<Uuid, Exercise>>

Remaps ids from updates to exercises that may have their ids regenerated.