2 Props mysteriously missing from the interaction widget
2 Props mysteriously missing from the interaction widget

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

This is roughly because we render a default interaction before the first
prop change comes from the editor; so we need valid defaultProps on the
widget instead of just the widgetEditor

Test Plan:
 * open `article.html`
 * add an interaction widget

Auditors: eater

sachgits/perseus,alexristich/perseus,learningequality/perseus,ariabuckles/perseus,iamchenxin/perseus,daukantas/perseus,learningequality/perseus,chirilo/perseus,chirilo/perseus,abe4mvp/perseus,iamchenxin/perseus,ariabuckles/perseus,ariabuckles/perseus,sachgits/perseus,ariabuckles/perseus,abe4mvp/perseus,alexristich/perseus,daukantas/perseus