Change how account balances are stored in the db to facilitate many currency types

[einsteinx2]

Right now we store account balances and transaction amounts in cents. That works for USD and other 2 decimal currencies, but doesn't work for things like bitcoin which have an indeterminate amount of decimal places. Also we make assumptions in the code when we do things like sorting by price that will need to take currencies into account.

For now, we don't need to figure everything out, since we aren't showing transactions yet, we just need a currency agnostic way to store the values for accounts that are still easily queryable.

[einsteinx2]

For now I've decided to just add a decimals field to define the number of decimal places and continue to store the balances as integers. We can think about this more in the future.

[einsteinx2]

Done