Remove robots.txt, favicon.ico; namespace static files

Django perms

cleaned a bit Response/ResponseMixin code, added some documentation + renamed ErrorResponse to ImmediateResponse

Ensure duplicate "page" parameters are not created

Previously, URLObject.add_query_param was used to generate
next/previous page links in PaginatorMixin. This resulted
in (for example) page 2's "next" link having the params:

   ?page=2&page=3

Instead, URLObject.set_query_param should be used to replace
the current value of the "page" parameter.