[#2750] Move create_country_codes() to a better place

Move example_idatasetform's create_country_codes() to a better place.
[#2750] Add setup_template_variables() back

Add setup_template_variables() back into ExampleIDatasetFormPlugin, just
counts how many times it gets called, for testing purposes.
[#2750] Use super()

Seems like the right thing to do