Translate non-string values in the `<Dropdown.Native>`

[caillou]

The <Dropdown.Native> abstraction is leaky. Currently, only string values work.

Ideally, users of the style-guide should not care about this, e.g.: https://github.com/orbiting/republik-admin-frontend/pull/59#discussion_r446255432

if (typeof value !== 'string') {
  // map a random ID to the value.
}