address some of #8308

Proposed fixes:

• Consistent display of draft state across dataset pages
• Error handling is now done on publish, so validators that only apply certain rules to fields for published datasets (e.g. required-only-when-published) will be properly displayed instead of causing a server error.
• templates/package/new_package_form.html content has been merged into its parent template templates/package/snippets/package_form.html and is now marked as deprecated

This PR changes the "Save" button on the current theme draft dataset editing interface to "Publish":

This brings it in line with the new theme:

It also adds the "Unpublish" feature (move a dataset back to draft state) to the current theme:

Already available in the new theme:

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport