Add system option to turn off sales ledger creation option

With the new sales ledger creation feature that has been brought in, because it may not be the sales team's responsibility to create sales ledgers, for example, customers may not want to have the sales ledger creation feature turned on so that the accounts team are able to create ledgers in the accounts system (following any credit checks processes they may need to internally) and make use of either the customer import or just the sales ledger import if it's for an existing customer who just doesn't have a sales ledger set up yet.

We should therefore have a system option (like we do for product creation) which allows customers to choose whether they want to use this functionality or not.