Explorer: increase visibility of decimal digits for currency values, where possible #1009

Mentioned in #965

For currencies where Siacoin converts to long decimal numbers, we display 0.00. BTC is an example. For cryptocurrencies, we may be able to look to their subdivisions (there's a question about symbols here), but fiat is a tougher problem. Where we can't convert/supply the value in a lower subdivision, we should make sure to surface it through a tooltip, where we can.