Allow additional arguments to listing() call.

The urls setup of old gave us various arguments at call time for
custom_index(), and some where useful. Hopefully, listing() can support
the same type of system, over time, once adapted. Here's an attempt to
get it started, first by supporting paginate_by.

Remove custom_index() method from news view.

I don't believe custom_index() should be used anymore given that we have
this new listing setup. At least, that's what I surmise based on
current error messages. I may bring this back if removing it breaks
something else.

Fix cut and paste error from documentation

This is trying to build a pagination of a news queryset.

HT tmarble for pointing this out.

Attempt to use Django 1.7 Paginator object.

Upon reading
https://docs.djangoproject.com/en/1.7/topics/pagination/#using-paginator-in-a-view

I wonder if we should simply follow this model for pagination of news
items. I've attempted to adapt that herein. This is a first attempt
and likely will have issues.

Adapted Django application stuff for Conservancy; and normalized to string
"conservancy" within code and templates.