ui: reflect chain sync progress of backend

[Roasbeef]

Depending on the exact configuration a user is using, it may be the case that after the UI is up, and the user has logged in, the backend is still syncing to the latest tip of the blockchain. In this scenario, certain lnd RPCs won't be available as we only fully start the RPC server once we know the backend is fully synced to the chain.

This has two implications:

1. We should show sync progress in the UI, and gray out certain actions until the backend has been synced.
2. We may want to wait off on starting the loopd portion of the binary (if it's running locally), as a recent change to lnd 0.10.1, makes the ChainNotifier RPC server no longer start up if the chain is still being synced.