Feature view on dataset page implemented

Fix for: #41

This PR includes following major changes:

Added additional extra field featured_view added on dataset schema in order to store the selected view as metadata of a dataset.
New tab View added on the dataset page that shows the list of available views and UI to select the view for featuring.
New endpoint /dataset/<id>/view and controller added to submit and view the list.
Template helper func featured_view_url added to generate the full URL to embed in an iframe.

Screenshot after Implementation: captures_chrome-capture-2022-4-20

datopian / ckanext-iaea