Replace empty BalanceOverviewCard with BalanceOnboardingCard showing
two steps:
1. Create an account
2. Enter a snapshot
Step 2 is grayed out until at least one account exists; the entire
card is replaced by BalanceOverviewCard once a snapshot is recorded.
Hide "+ New snapshot" button when 0 accounts (it lives inside the
overview card, which is now hidden in that state).
Improve SnapshotEditPage noAccounts copy to clarify account vs
snapshot semantics.
Resolves#178
