[#713] Blockify package_basic_fields.html

Add a package_basic_fields_custom block to package_basic_fields.html,
which IDatasetForm plugins can use to add custom fields to the first
page of the new package form without duplicating any template code.
Update example_idatasetform to use this new block for one of its custom
fields.

Fixes #713.
Merge pull request #713 from okfn/713-blockify-package-basic-fields.html

Blockify package_basic_fields.html