Use Facebook app/page data from local settings

Add Facebook app's ID and Facebook page's namespace/username to local dist settings file. Send them through to be used in Download Tab's JavaScript.
Add URL that redirects back to Facebook page

Redirects back to Facebook page and conserves any GET parameters by sending them through `app_data`. Utility function is added to convert the GET dictionary into an `app_data` query string. Lazily evaluated setting is added for Facebook page's URL.
Refactor Facebook js class to not use JS SDK

By sending the `useUrlDialogs` option, the class doesn't load the Facebook JavaScript SDK and will prepare to use direct URL dialogs rather than `FB` dialogs.
Remove old Download Tab grid placeholder image
Add dialog capabilities to Facebook js class

Works for both JavaScript SDK and direct URL dialog options. Initially only `share` is part of the "public" API.
Add Facebook share to Download Tab

Share is triggered by the second scene's "Share" button. When redirected back to the tab from the direct URL dialog, the third scene, "Get Involved", is rendered.