Allow multiple invoices per month

[aparcar]

Currently the invoices are created via <year>.<month> however a better approach would be to have <year>.<month>.<counter> to allow additional invoices like follow ups or something

[olf42]

Agreed!

Maybe we can also refactor the whole invoice generation process s.t. various numeration schemes are possible, e.g.

1. continuous: nnn
2. year.continuious: 2019.nnn
3. customer.year.continuos: XXXX.2019.nnn
4. customer.year.month.continuous.nnn

For westnetz, No. 3 with additional prefix on the number would be desired, e.g. XXXX.2019.Q2