Move User to flipper-common

Summary: Move User from reducers to flipper-common. User will now be usable by other modules.

Reviewed By: passy

Differential Revision: D37599802

fbshipit-source-id: 66412e7ed00bf27448fa2deae70f0e8e80303aba

desktop/flipper-common/src/index.tsx

@@ -64,3 +64,4 @@ export * from './PluginDetails';
 export * from './doctor';
 export * from './ServerAddOn';
 export * from './transport';
+export * from './User';