Add "shaded area bounded by polar curve" template

Summary:
Factor out polar plot setup template
Fix up existing polar examples to better fit shared template

Test Plan:
Run ./app.py
Hit http://localhost:5001/#42
Convert to image and get: https://ka-perseus-graphie.s3.amazonaws.com/e8aae2b75944bf365a6ddbc78d953117d41f90f9.png
Hit http://localhost:5001/#29
Convert to image and get: https://ka-perseus-graphie.s3.amazonaws.com/ca782cb5e9eabb076ee1f2f9c8a2b810f17ffec0.png
Hit http://localhost:5001/#29a
Convert to image and get: https://ka-perseus-graphie.s3.amazonaws.com/0cd0a70b74757b2cc1a868c7b3c854e77671e253.png
Go back to http://localhost:5001/#42 and edit it in various fun ways, resulting in things like:
https://ka-perseus-graphie.s3.amazonaws.com/455a71193875a07b64b2f983859339568c90d180.png

Reviewers: charlie

Reviewed By: charlie

Maniphest Tasks: T2712

Differential Revision: http://phabricator.khanacademy.org/D10296