Add a configuration setting for a combined wallet

[renbinden]

Some admins want /wallet to show a combined wallet with all currencies.
This should be configurable (such that people who use the current behaviour aren't surprised by the changes).
Some extra consideration needs to made around payments and such to make sure new stacks could fit in a combined wallet - rather than checking against the constant 1728 value, we need to make sure the amount fits in a stack of that currency.