Allow extending polymorphic models in plugins

[OmeGak]

Subclassing polymorphic models is not currently allowed from plugins. Example:

class RegistrationFormSessionsSection(RegistrationFormSection):
    __mapper_args__ = {
        'polymorphic_identity': RegistrationFormItemType.section_sessions
    }

Blows up like this:

Exception: Plugin 'un' added a model which is not in a plugin schema ('RegistrationFormSessionsSection' in 'event_registration')

This PR solves it.

[ThiefMaster]

I think our check in _import_models doesn't even work all the time, since usually the model already got imported when loading the plugin.. anyway, for the cases where it does work your change looks reasonable :)