Create npm package out of client's api

[unDemian]

Built on top of #180

Summary

This PR prepares happychat-client to be exposed as a npm package. Paths are resolved using babel-plugin-module-resolver and configuration json files are inlined using babel-plugin-inline-json.

CSS url is moved to a configuration field and it will be server by webpack even when developing the npm package.

Testing

1. Switch calypso to try/happychat-client branch (Use happychat-client npm package wp-calypso#23630)
2. Link this package to wp-calypso, by following these steps

• run npm link in this folder
• run npm link happychat-client in calypso folder

3. Run npm run dev:standalone in this repo
4. Grab your oauth token from a wpcom request made by the client (http://localhost:9000) it is under Authorization header after Bearer
5. Add the token to calypso https://github.com/Automattic/wp-calypso/pull/23630/files#diff-56de7193249f7b0107d6033c1a7737acR22
6. Run npm dev:npm
7. Access http://calypso.localhost:3000/me/chat
8. It should load the happychat-client iframe

[unDemian]

Closed in favor of smaller PRs
#183
#184