Persist settings in ${XDG_CONFIG_HOME}/flipper/settings

Summary:
Moves the settings state from electron local storage into a json file in the users configured config location.
Unless modified by the user, this will usually be `~/.config/flipper/settings.json`

Settings will now persist across re-installs, and can now be easily inspected and backed up.

Reviewed By: passy

Differential Revision: D17712687

fbshipit-source-id: 1e778063e41d0a1a86145817b9797bf0458121da

src/chrome/SettingsSheet.tsx

@@ -163,6 +163,6 @@ class SignInSheet extends Component<Props, State> {
 }
 
 export default connect<StateFromProps, DispatchFromProps, OwnProps, Store>(
-  ({settingsState}) => ({settings: settingsState.settings}),
+  ({settingsState}) => ({settings: settingsState}),
   {updateSettings},
 )(SignInSheet);