Overwrite searchableAs in ElasticSearchable
Overwrite searchableAs in ElasticSearchable

I had to do this because `images` were being indexed as `assets`.

This also fixes the following issues:

  - Mappings were defined w/ kebab_case, but model instances were being
    indexed via snake_case, which also results in duplicate types, e.g.
    `tour-stops` vs. `tour_stops` + `shop-products` vs. `shop_products`
    The former variants were intended, but always ended up empty.

  - Polymorphic models index into the proper types, e.g. Artists index
    into `artists` now, not `agents`. Same thing for Asset subtypes.

art-institute-of-chicago/data-aggregator,art-institute-of-chicago/data-aggregator,art-institute-of-chicago/data-aggregator