[#1792] Pass the filters to iframed resource views

It works simply by passing the `filters` query string to the iframed URL. As
iframed resources don't add JS files outside of the iframe, and we need
`ckan.views.viewhelpers.filters` to create the filters, I needed to move that
core from `ckanext-viewhelpers` to core and load it.

In the future, `ckanext-viewhelpers` will be completely merged into core and
will be removed.
[#1792] Configure Recline's previews as filterable