create a has and belongs to many association for spree taxons and prototypes

add taxons for admin prototypes

add spec to prototypes feature for deleting

remove old `.find_all_by_prototype` code

This code seems old. It is trying to do a table join on a non-existent
relationship, or rather, a table that is non-namespaces with `spree_`.
Also, I couldn’t get it working while writing tests for it. Tested out
the method on a current project and returned PG errors. Tests returned
SQLite errors.

Fixes #5476
Remove shoulda matchers again.