Add basic callout support to levelbuilder (defined via JSON)

I'm wondering if this could be refactored so the `.select`, `.reject` and `.each` down here are used for *both* the custom and non-custom ones (i.e., single-point-of-truth this logic).

One way,

1. have a new variable `available_callouts` that can be either `JSON.parse.map{create callout}` OR the `Callouts.where(script_level: @script_level)`
2. call the `.select`, `.reject`, `.each` on that intermediate collection, set *that* to callouts_to_show

(that would create a few extra intermediate Callout objects in the case of having seen a level builder one, but that's not a significant amount of data)