Force Spree.user_class#spree_roles to use has_many instead of HABTM
Force Spree.user_class#spree_roles to use has_many instead of HABTM

This ensures that accessing admin areas works correctly regardless of
which tenant is active. Previously `#spree_roles` was handled via HABTM
association and was therefore causing queries on the spree_users_roles
table to never be scoped to the public schema.

groundctrl/shopping_mall