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/Layout.tsx

@@ -198,7 +198,7 @@ const SandySplitContainer = styled.div<{
   flex: 1,
   flexDirection: props.flexDirection,
   alignItems: props.center ? 'center' : 'stretch',
-  overflow: 'hidden',
+  overflow: props.center ? undefined : 'hidden', // only use overflow hidden in container mode, to avoid weird resizing issues
   '> :nth-child(1)': {
     flex: props.grow === 1 ? splitGrowStyle : splitFixedStyle,
     minWidth: props.grow === 1 ? 0 : undefined,