Date of invoicing activity changed to new date if activity date is also moved

[Jurak-Lucais]

the dies diner was moved by a week but the invoicing was still considered to happen on the original date, which is a problem because we were going to adjust the prices later since the activity shouldn't have fallen within the invoice date selection but then this occured :/
-Tristan

[Kurocon]

Related to / Duplicate of #56 , #314 and #687