Apply `futurize --stage-2` Python 2/3 compatibility transformations.

These changes specifically require the use of the "future" library.

Migrate RequestContext users to render() calls.

This was necessary because render_to_response no longer takes a
context_instance. We were always using this to pass in a RequestContext,
which the render shortcut can already do, so switch to that.

supporter: Support donors upgrading to supporters.

With a valid URL, a past donor can upgrade to a supporter by donating
the rest of the amount. Django validates the data and, if it passes,
puts it in the form. The JavaScript does most of the work from there.