Add `getBaseUrl` method to Download Tab's App class

Returns base URL generated from the `window.location` object. Can receive `options` object to include the protocol or send in a custom protocol. App constructor now receives the `window` object to gain access to `location`.
Add channel URL for Facebook JavaScript SDK
Send server-side data to Download Tab's singleton

Sent via script tag in the template. Singleton initialization is now performed in the script tag rather than in Base.js. Server data is now injected into classes that inherit `Base` via the `_initData` property. Initial server data is channel URL for Facebook JavaScript SDK and `DEV` setting for the front end to be aware of development mode.
Add Facebook JavaScript SDK to Download Tab

Adds a new `Facebook` wrapper class for asynchronous loading of the SDK and initialization code. The process begins as soon as the instance is created unless the `startImmediately` option is sent a falsy, non-`undefined` value.

The `App` class creates an instance of `Facebook`, runs it immediately and sends in a callback that makes use of the SDK.