Set the networks a business uses

Replaces the networks in use and the default network for every member and API key of the business. Requires networks_manage in enforced mode or permissions_manage in legacy mode. A change applies immediately, with no two-factor code; turning a network on grants no rail. Turning off a network that holds funds or has pending transactions is refused with network_holds_funds, and with network_holdings_unavailable when what it holds cannot be checked; a network the business is not eligible for with network_not_eligible, and an invalid set with invalid_networks. Saving what is already saved changes nothing and records nothing.