Add option to show account balance instead of available funds

Some foodcoops do not use the accounting feature of the foodsoft, which
results in wrong calculation of the available credit. To avoid
confusions show the current account balance instead.

app/views/admin/configs/_tab_payment.html.haml

@@ -10,6 +10,7 @@
   .input-prepend
     %span.add-on= t 'number.currency.format.unit'
     = config_input_field form, :minimum_balance, as: :decimal, class: 'input-small'
+= config_input form, :charge_members_manually, as: :boolean
 = config_input form, :use_iban, as: :boolean
 
 %h4= t '.schedule_title'