create/update models for many-to-many relationships do not happen if either side is read-only

[zachrybaker]

Scenario:

I have a role table, and a user table, and a userrole table that represents the many-to-many relationship, and I specify role as an entity to exclude via the model:create:exclude:entities and model:update:exclude:entities lists.

Expectation:

Generation would create models for create and update UserRole, since I have not excluded UserRole, just Role.

Actual:

I have to remove the excludes for Role create/update to get UserRole create/update.

It is sensible in the real world to CRUD the many-to-many relationship without CRUDing one side or the other.