Implement filter to apply in *ngFor

[kilchenmann]

I was wondering if I can combine *ngFor with *ngIf somehow. The combination is not allowed and ends in an angular error. But in some cases it would be good to combine them to avoid empty containers. I found a solution to do that: https://stackoverflow.com/questions/34164413/how-to-apply-filters-to-ngfor
I will create a new pipe in @knora/action