#281 updated billing json
#281 added new product to alkis_products.properties
#281 new dialog class for choosing the stichtag for new product
#281 bundle file for stichtag chooser dialog
#281 integrated new bestandnachweis product in buchungsblattrenderer
#281 added new product in buchungsblatt aggregation renderer
#281 validating stichtag date against server date
resolved merge conflict in billing.json