Convert lots of exercises that use regex to use expression entry

Summary:
Needed to make these exercises work well in the iPad app.
TODO(eater): Plug our wysiwyg embuttoned expression editor into khan-exercises

Test Plan:
Do `grep regex exercises/*` and see that just exploring_angle_pairs_1 remains,
which we're ok with.

Run the unit tests at http://exercises.ka.local/test/ to verify the fixes to
expressions in multiples/sets work and nothing else got broken in the process.

Attempt one problem per problem type for each of the modified exercises,
entering the question as the answer and get some sort of message about not
simplifying or not being in the right form. Enter the answer from the hints and
get graded correctly.

Cross fingers that users will be able to figure this all out :\

Reviewers: charlie, aria

Reviewed By: aria

Subscribers: aria, peter

Differential Revision: http://phabricator.khanacademy.org/D12070