How react errors are destroying particles, and could set off nuclear reactions

Summary:
Be more lenient about prop types for ArticleEditor, given that
`Editor` and `Renderer` are reasonably happy about defaulting
content/widgets/images.

This change makes it safe to initialize a Particle with the
json "{}", which is wonderful for exposing a simple interface
to webapp.

Test Plan:
 * `make subperseus`
 * Make a new perseus article on the particles-draft branch
   in webapp
 * see no react alerts

Auditors: alex