Only clear autosave on successful save.

There is a cookie set on the client side that is modified on the server side
upon successful save.
Set cookie paths to be specifically for one page.
Set cookie path on server side.

Django appears to be naive about request cookies (not sure if it's django or
the way that cookies work in general). We need to explicitly set the path to
the referer, otherwise we end up with two cookies called "autosave_success."
Bump to v0.7.0.