Check for unscoped model finds

Adds a simple check for unscoped calls to `#find` on models that belong to other models. For example, if `Email` belongs to `User`, calling `Email.find params[:email_id]` should probably be `some_user.emails.find params[:email_id]` or similar.

The confidence on this will always be low since this kind of find may be intentional. It may be too false positive heavy to be worthwhile.

/cc @gregose @ptoomey3 
This should probably do some filtering of known safe stuff (constants, symbols) and/or check for user input.