Use db_to_form_package_schema() by default

Make DefaultDatasetForm's db_to_form_schema() method return
db_to_form_package_schema() rather than None, so that validation against
db_to_form_package_schema() is done by default, even when there's no
IDatasetForm plugin active.

The reason for this is that bugs in db_to_form_package_schema() can
cause pages to crash, features to stop working, etc., but the tests
currently do not catch these bugs because db_to_form_package_schema() is
only used when an IDatasetForm plugin that returns a schema based on
db_to_form_package_schema() is active, and no such plugin is active when
the tests run.
Show vocab tags as normal tags by default

Commit 77663865a changed the default behaviour of vocab tags so that
they are _not_ shown as normal tags (e.g. on the dataset read and search
pages). I thought this was a good idea because it's weird when you use
tag vocabulary to add a custom field, to see that tag also appear in the
normal tags. However, changing this behaviour breaks some tests, so I'm
changing it back!

If an extension is using a tag vocabulary to add a custom field, and it
doesn't want tags from that vocabulary to appear alongside normal tags,
then the extension can add the freet_tags_only converter to the schema
itself.