This PR ensures that when you're generating a scaffold or model and that
model should belong to another database it will create an abstract class
if it doesn't already exist.

The new abstract class will ensure that the new model inherits from that
class, but will not be deleted if the scaffold is deleted. This is
because Rails can't know if you have other models inheriting from that
class so we don't want to revoke that if the scaffold is destroyed.

If the abstract class already exists it won't be created twice. If the
options for `parent` are set, the generator will use that as the
abstract class instead of creating one. The generated abstract class
will add the writing connection automatically, users need to add the
reading connection themselves as Rails doesn't know which is the reading
connection.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>