Facebook Download Tab

Adds Download Tab under `/facebookapps/downloadtab/` URL. The tab has the same "Download Firefox" button as `/firefox/new/`. After downloading, the scene changes to reveal a button for sharing the tab on Facebook. Once the user shares, a third scene is revealed with cross-promos for Affiliates and Facebook Messenger for Firefox and a button to invite friends on Facebook. All Facebook dialogs use direct URLs rather than the Facebook JavaScript SDK (see [https://developers.facebook.com/docs/reference/dialogs/](https://developers.facebook.com/docs/reference/dialogs/)). Server-side redirects are used to take users back to the tab after a dialog (the `/facebookapps/tab_redirect/` URL).

Two new local settings were added to `settings/local.py-dist`. Both are required by the Download Tab for Facebook integration. These are `FACEBOOK_PAGE_NAMESPACE`, the username/namespace of the Facebook page on which the tab is to be placed, and `FACEBOOK_APP_ID`, the app ID for the Download Tab Facebook app. For configuring the app on Facebook, the Page Tab URL should be `/en-US/facebookapps/downloadtab/` (remember to set both secure and insecure versions), Page Tab Width Normal (`810px`) and the App on Facebook Canvas URL should be `/en-US/facebookapps/tab_redirect/js/` (again set both secure and insecure). Also remember to set the correct App Domain.