Change replaced_by to ManyToOneField in django-orghierarchy and fix filters accordingly

[Rikuoja]

Currently, an organization may replace one old organization.

If we want the same organization to replace several old organizations (@aleksisalonen will have to decide on that), django-orghierarchy needs to be updated, along with the organization filters in api.py.

[Rikuoja]

As the only cases needing this are Ympäristökeskus, Rakennusvalvontavirasto, Rakennusvirasto (Kaupunginympäristön toimiala already replaces Kaupunkisuunnitteluvirasto), Varhaiskasvatusvirasto (Kasvatuksen ja koulutuksen toimiala already replaces Opetusvirasto) and Tietokeskus (Kaupunginkanslia already replaces Kaupunginkanslia), I suggest it is easier to just transfer a few users by hand to the new organization, than start coding this feature separately.