proptype validation too strict

[mbohal]

In the following components:

• CTA
• Card
• FormLayout
• Media
• Tabs

we validate the children prop with:

  children: PropTypes.oneOfType([
    PropTypes.element,
    PropTypes.arrayOf(PropTypes.element),
  ]).isRequired,

This is too strict, because it fails when false is passed in. This is common with idomatic code such as:

<CTA>
  {condition && <CTAStart />}
</CTA>

We should probably use children: PropTypes.node.isRequired to revert it to the state before #301 was merged.

Since propType validation is off in production builds, this has no effect on production code.