Deprecate old facet data structures and related functions.

[icmurray]

In branch feature-1821-multilingual-extension, a new faceting datastructure was introduced. This makes the old one and related functions obsolete. These have been marked as deprecated for the 1.7 release, and should be removed for the 1.8 release.

• helpers.py:facet_items()
• facets.html:facet_sidebar()
• uses of c.facets (rather than c.new_facets)

[seanh]

Note that c.new_facets has now been renamed c.search_facets.

Before this ticket is closed the old c.facets should also be removed from the template context. It's put there by ckan/logic/action/get.py:package_search(), which also puts the new c.search_facets there.