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/flipper-server/server.js

@@ -1,3 +1,4 @@
+#!/usr/bin/env node
 /**
  * Copyright (c) Facebook, Inc. and its affiliates.
  *