Multi-ledger transactions must take into account ledger currencies

[rgalanakis]

We need to explicitly handle it, even if it is just erroring for now if we see ledgers with mismatched currencies.

The main use case for this in the short term is scrip/discounted dollars- so for example, if you load $10 into your account into Scrip (&), you get &10, and the conversion rate is $1.1 to &1, so when you take a $5 scooter ride, we book -&4.5 or whatever. We can also use this for true multi-currency, but that is way outside of scope for now. Real scrip is also a bit outside scope, so make this explicitly unsupported.