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/index.ts

@@ -34,6 +34,7 @@ export {
 
 export {theme} from './ui/theme';
 export {Layout} from './ui/Layout';
+export {NUX, NuxManagerContext, createNuxManager} from './ui/NUX';
 
 // It's not ideal that this exists in flipper-plugin sources directly,
 // but is the least pain for plugin authors.