Scheming support

Adding package_type as a variable passed to package templates and group_type to Group/org templates. This lets ckanext-scheming do its custom edit form and view thing.

Also planning to clean out some old code while I'm digging around the package controller.
Oh, I had added that so that updates can continue to be validated against their original type (not the new type value passed)

That might not actually be a good idea, and it looks like the current code will still allow changing the type.. possibly creating an invalid dataset because the other validation rules might have been used.

The current controller code looks up the plugin to be used based on the type stored in the db not the type passed.

What do you think? Should changing the type be just disallowed?