Supply ASSET_HOST to webpack for internal asset compilation

[gauravtiwari]

Related to #208

With CDN

 {
"app.js": "//d309ficnxxdltp.cloudfront.net/entries/app-15c9cd01cb8b89b930bf.js",
"lacuna-webfont.woff": "//d309ficnxxdltp.cloudfront.net/entries/lacuna-webfont-0eb6add367ef5fc69c4569894525bd52.woff" 
}

Without CDN

{
"app.js": "/entries/app-dd8750c877fb3fcd1082.js",
"lacuna-webfont.woff": "/entries/lacuna-webfont-0eb6add367ef5fc69c4569894525bd52.woff" 
}

[gauravtiwari]

@schovi Please could you review this PR?

[schovi]

Awesome 👍

[gauravtiwari]

thanks for reviewing @schovi 🍰 🍫

[tarr11]

This breaks javascript_pack_tag if asset_host is not prefixed with a protocol. Note: You have to run rake assets:precompile to generate a new manifest to test this bug. It occurs because the manifest contains the CDN path, and will not happen if you do not precompile the assets (and associated manifest.json)

Breaks:

config.action_controller.asset_host = 'assets.example.com'

Generates malformed URLs:

 <script src="http://assets.example.com/javascripts/assets.example.com/packs/application-58fb4673fe4502e4714c.js"></script>

Works:

config.action_controller.asset_host = '//assets.example.com'

Generates correct URLs:

 <script src="//assets.example.com/packs/application-58fb4673fe4502e4714c.js"></script>

Since most of the examples and scaffold code is of the non-protocol form, this should probably be fixed.
http://api.rubyonrails.org/classes/ActionView/Helpers/AssetUrlHelper.html