MBL-2075: Add Backings dashboard as its own tab

[amy-at-kickstarter]

📲 What

Add Backings Dashboard as its own tab in the root tab bar controller.

Logged Out	Logged In

🤔 Why

This is part of the UI updates for PPO V2!

🛠 How

1. Add new RootViewControllerData type, .backings, and delete old type, .pledgedProjectsAndActivities
2. Refacto ActivitiesViewController to place an EmptyStatesViewController directly in the tab bar when the user is logged out. Then do the same thing for PPOContainerViewController in its new tab. Adding the empty state controller directly to the tab bar ensures the layout is identical between the Backings and Activity tabs.
3. Update PPOContainerViewController to remove the top paged controller
4. Temporarily disable badging on the PPO tab. Badging will be addressed in a follow-up ticket (MBL-2151), to keep this manageable.