views: Use messages framework in 'patch'

views: Use messages framework in 'patch'

Django provides the messages framework as part of the core library.
This framework allows for "toast"-style notifications to the user,
for things like post-form processing notifcations. At the moment
patchwork provides this functionality using custom code. However, this
code is not well tested and, like any code, incurs some degree of
maintenance overhead. It would be easier to use the "batteries" that
Django provides so begin doing just that.

This change only covers one of two places in which this custom
messages framework is currently used. By extension, it also covers one
of the two places in which 'render_to_response' is still used. This
"other place" will be addressed in a follow-up commit.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Tested-by: Andy Doan <andy.doan@linaro.org>