Added a template for plotting slope fields

Summary:
Users can now provide a first-order differential equation (i.e., a function of x and y)
and plot its slope field on a graph, specifying the density of the field and the length of the
slope lines. Fixes T2687.

Test Plan:
- Run ./app.py
- Go to http://localhost:5001/#41 and see the slope field for a first-order differential equation.
- Play around with xStep, yStep, and slopeLength to see effects. xStep and yStep should determine how frequently the slope is calculated, while slopeLength should determine the actual length of the slope lines.
- Use this useful [tool](http://www.mathscoop.com/calculus/differential-equations/slope-field-generator.php) to compare results.

Reviewers: eater

Reviewed By: eater

Subscribers: jack

Maniphest Tasks: T2687

Differential Revision: http://phabricator.khanacademy.org/D10259