Adds problem numbers to group widgets.

Summary:
This allows us to display problem numbers when a problem has multiple
sub problems within itself (contained within group widgets).

Test Plan:
Viewed third problem of
http://localhost:8080/mission/sat/task/5452478162141184 with a hacked
in problem number annotation function and observed incrementing problem
numbers within the problem.

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D14296