Introduce NUX element

Summary:
allow-large-files

This diff introces the `NUX` element that can be wrapped around any other element to give a first-time usage hint.

Hint dismissal is stored by taking a hash of the hint contents, and scoped per plugin.

Users can reset the 'read' status in the settings page

Example usage:

```
<NUX
  title="Use bookmarks to directly navigate to a location in the app."
  placement="right">
  <Input addonAfter={<SettingOutlined />} defaultValue="mysite" />
</NUX>
```

Reviewed By: nikoant

Differential Revision: D24622276

fbshipit-source-id: 0265634f9ab50c32214b74f033f59482cd986f23

desktop/flipper-plugin/src/ui/theme.tsx

@@ -11,6 +11,7 @@
 
 export const theme = {
   white: 'white', // use as counter color for primary
+  black: 'black',
   primaryColor: 'var(--flipper-primary-color)',
   successColor: 'var(--flipper-success-color)',
   errorColor: 'var(--flipper-error-color)',