Add size property to DetailItem

[wpwood]

We should add a size property to the DetailItem component, like we have on Form. This allows for PropType.oneOf validation, where just passing a className does not.

Possible values for the class would be:

const SIZE_CLASSES = {
  'xsmall': 'rs-detail-xsmall',
  'small': 'rs-detail-small',
  'medium': 'rs-detail-medium',
  'large': 'rs-detail-large',
  'xlarge': 'rs-detail-xlarge',
  'xxlarge': 'rs-detail-xxlarge'
};