Switch from webapp/django to webapp2/jinja2.

Summary:
It's better because of all those '2's!

The motivation is that using djano (via google.appengine.ext.template)
is deprecated in more recent appengine sdk's.  Instead you're supposed
to import a template library explicitly.  I decided to move to jinja2
since it's simpler and I don't need all the stuff django has.

In a similar vein, instead of using ext.webapp, I explicitly import a
webapp engine, in this case webapp2, which knows how to work happily
with jinja2.

The only change required was I had to add my own filters for the
dates, since jinja2 doesn't have date-formatting filters the way
django does.

Test Plan:
Ran a dev-appserver, and visited
   http://localhost:8080/weekly?week=09-01-2014
and registered as a user and entered a snippet, and everything seemed
to render correctly.

Also,
   ./snippets_test.py
passed without error.

Reviewers: chris, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12886