re-style asset form
This PR consists of a number of commits which add functionality followed by braking apart the current asset form and re-styling it.
49084131 breaks the existing AssetForm and AssetFormHeader component into sub-components. We split these components into "active" container components and pure "presentational" components.
The other large commit is 53aa57d0 which provides a "withDraft" higher order component which allows connecting components to the current draft being edited and provides callback functions to allow the draft to be modified by the component.
Closes #16 (closed).