Fix widgy_mezzanine preview for Mezzanine==3.1.2

With the introduction of mezzanine.pages.context_processors.page, there
was a change in how PageMiddleware and the page view interact which
broke the PreviewView and HandleFormView in widgy_mezzanine.  This
commit fixes the problem in a way that works for both pre- and
post-3.1.2 Mezzanine.

See https://github.com/stephenmcd/mezzanine/commit/a28cdf41643101b36497da817212081d2f3a4d3b for more details about the change in mezzanine.