We have just over 50 duplicate implementations of the method
`find_object` in our graphql mutation classes.

This change replaces these by updateing the existing default
implementation, which raises a `NotImplementedError`, with the
behaviour from the duplicate implementations which is by
definition the default implementation.

This change also removes some superclasses and one module whose only
function was to implement `find_object` in the same way.

In some cases the method signatures were `find_object(id)` rather than
`find_object(id:)` - note the non keyword argument.  In those cases the
caller (`find_object` is called with argument pass through from
`authorized_find!`) has been updated.