Make billing period more flexible: e.g. move billing to this month if it is at end of last month

[Taimonania]

Right now it is not possible to always move the billing period to this month for e.g. income that sometimes arrives at 29. of last month and sometimes on 2. of this month.

Two possible approaches:

1. Add a field "start of month" that states when the month for this billing period should start
2. Add possibility to move billing date x days/months/years and separately set whether it should be billed daily/monthly/yearly