Make flipper-server stand-alone servable

Summary:
This diff makes flipper-server stand-alone servable as a single package.

It basically works as follows:

- (default) plugins are build as usually into static folder
- static folder is copied into flipper-server/static (as far as relevant)
- `flipper-server/src/index.tsx` is bundled into `flipper-server/dist/index.js`
- `flipper-ui-browser/src/index.tsx` is bundled into `flipper-server/static/bundle.js`

If flipper-server is started without the `--bundler` config, the `bundle.js` will be served statically, rather than starting up the Metro bundler with all the typical transforms (in a distributed version of the package those all wouldn't resolve)

Things to be done in next diffs:

* make sure plugins actually load in the non bundled setup
* make sure flipper-server gets published properly
* make sure build is created as part of CI

At the end of this stack, running `npx flipper-server` on any machine should basically be a viable approach to get flipper up and running locally :)

Reviewed By: nikoant

Differential Revision: D33171107

fbshipit-source-id: 4af8ac2699467a0b55283fe084640482700744c2

desktop/scripts/paths.ts

@@ -14,6 +14,7 @@ export const appDir = path.join(rootDir, 'app');
 export const browserUiDir = path.join(rootDir, 'flipper-ui-browser');
 export const staticDir = path.join(rootDir, 'static');
 export const serverDir = path.join(rootDir, 'flipper-server');
+export const serverStaticDir = path.join(serverDir, 'static'); // for pre-bundled server, static resources are copied here
 export const defaultPluginsDir = path.join(staticDir, 'defaultPlugins');
 export const pluginsDir = path.join(rootDir, 'plugins');
 export const fbPluginsDir = path.join(pluginsDir, 'fb');