Add polar plot with tangent line example

Test Plan:
Run ./app.py
Hit http://localhost:5001/#29a and see a polar plot with a tangent line.
Click "Convert to image" and get a PNG version.
Hit http://localhost:5001/#29 and see a polar plot without a tangent line.
Hit http://localhost:5001/ and see a blank editor and no js errors
Hit http://localhost:5001/# and see a blank editor and no js errors
Hit http://localhost:5001/#invalid and see a blank editor and no js errors

Reviewers: charlie

Reviewed By: charlie

Differential Revision: http://phabricator.khanacademy.org/D10239