One mistake that ruined every grapher widget that ever lived in an article

Summary:
Use defaultProps on the grapher widget to make it not crash when adding
it to an article.

This is roughly because we render a default grapher before the first
prop change comes from the editor.

Test Plan:
 * open `article.html`
 * add a grapher widget

Auditors: charlie
2 features you should be careful to not disable

Summary:
Pass EnabledFeatures through article editors; default to
using mathquill and tooltips.

Test Plan:
Add an expression to article.html

Auditors: alex