Fix Intermediary Balance

[geoknee]

I cherry-picked some work from #111 for this.

• removes the intermediaryBalance protected member of the StateChannelWallet class.
• replaces with a getter which computes the balance from the latest state. This is the off chain source of truth about the intermediary balance
• removes code trying to read intermediary balance from the chain
• removes any bigint => Number casts from the clients. This happens in the UI now, but that can be fixed easily later on.
• DRYs out the UI quite a bit by introducing a useBalances hook

[geoknee]

before:

after: