[UI Framework] Reactify AssistedInput component

[cjcenizal]

This component will need to accept an assistance prop into which any element, e.g. a <KuiLink> instance, can be passed. We'll also need to support a padding prop, so we can make sure the text doesn't overlap the assistance.

<KuiAssistedInput
  assistance={<KuiLink href="#">Help</KuiLink>}
  padding="20px"
  onChange={this.onChange}
  value={this.state.value}
/>

[cjcenizal]

Focusing on EUI.