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