3/5 = 0.6 Add validation on room-node-installation to prevent overlapping installations.

[uliSchuster]

The M:N relation between a room and the installed node has its own relation model that allows for time-sliced attribution. Because the Django ORM does not support temporal SQL extensions, there is no DB integrity check that would prevent a node being attributed to more than one room at a time. Therefore, we need to introduce a validation higher up in the stack.
This validation could happen in

• the serialiser validation; this would be the earliest possible validation, and would need to be applied for all serialisers that alter node-to-room attributions;
• in custom managers on the models involved. It is not quire clear to me if we would need validations on both sides of the relation (the Node and Room models), or just on the RoomNodeInstallation relationship model.