Let people start entering in the new week's snippets starting on monday.

Summary:
Now, when you look at snippets on a monday, you're offered to enter
snippets for the week that has just started (even though not much has
happened yet).  This is for people who like to enter snippets as they
go along.  For those people who wait until monday to fill in the
previous week's snippets, they can still do so; the second snippet box
will be marked 'DUE TODAY'.

I also took the opportunity to upgrade to the python2.7 runtime, so
appengine doesn't yell at me anymore.

Test Plan:
snippets_test.py

I will also look at the console next monday to make sure it's ok.

Reviewers: chris, marcos

Reviewed By: marcos

Subscribers: khirasaki

Differential Revision: http://phabricator.khanacademy.org/D9264