Use the TokenBalancesController to track token balances

[Gudahtt]

The extension currently tracks token balances in the UI. Instead it should use the shared TokenBalancesController to track token balances in the background. This lets us avoid making separate network requests in each window, saving on network bandwidth.

The UI token balance tracking is clever about only being active when the balances are actively being viewed though. We should investigate ways to ensure network traffic does not increase with this change (i.e. only ask for balance updates when they are needed).

This relates to MetaMask/core#700

[bergeron]

I'm working on this