Check invoicing transaction dependencies

I implement methods in InvoicingTransactionTestCase which test quotes,
sales orders, and invoices whether they are in the state as created by
the methods prepareQuote, prepareSalesOrder, and prepareInvoice,
respectively.

Furthermore, I implement validation of input error messages if the user
does not enter values into the only items row in invoicing transactions.
I also hide the remove button if there is only one row in the items list
of invoicing transactions.

At last, I move stylesheet code for price tables from main.less to
invoicing-transaction.less and include the latter one in the sales items
forms.