[#415] db_to_form_package_schema() don't strip keys from resource dicts

Add several keys to the 'resources' subschema in
db_to_form_package_schema() so that validation doesn't strip the keys
from the resources dicts. This fixes a test that was failing if run with
an IDatasetForm plugin active, and may also fix some template crashes
when an IDatasetForm plugin is active if people are accessing these
resource keys in their templates.

Fixes #415.
Remove free_tags_only from db_to_form_package_schema()

Remove the free_tags_only converter from db_to_form_package_schema's
tags subschema.

This fixes a couple of tag vocabularies tests that were failing.

Not sure if this at the same time breaks something that we don't have
tests for, we may have to put this back in later if we find a problem.
Delete some unhelpful comments

Not clear what these comments mean or whether it's still true.
navl.validate: unflatten before restoring empty lists
navl.validate: fix key names in restored empty lists
[#375] Add license_title back to db_to_form_package_schema()

Looks like this got removed by accident, so license titles weren't
showing on dataset pages.
Revert "Remove free_tags_only from db_to_form_package_schema()"

This reverts commit aaa10bbff6e4c704e58f651f13b5b18cfa0a2571.

The free_tags_only converter is needed after all, because it prevents
vocab tags from appearing as normal tags on the dataset read and search
pages.

Reverting aaa10bbf means that a couple of tests are failing, but I think
it's the tests that are wrong not the code.