Planning overview colours Bestelbon and Akkoord are inverted

[heynist]

When an Akkoord has also a Bestelbon, it should be shown visually.
The current implementation hides a Bestelbon when an Akkoord period overlaps it.
It should be the other way around, since it will happen more frequently that we have an Akkoord (blue) but not a Bestelbon (green) yet.

[heynist]

More information gained during testing and comparing => The color-inversion is not always the case apparently: on the 4C instance (http://ec2-52-29-142-143.eu-central-1.compute.amazonaws.com/) we see Bestelbon clearly taking visual precedence over Akkoord, but on the 2TP instance (http://ec2-52-58-191-63.eu-central-1.compute.amazonaws.com/#/planning) for Bert Duerinck we see no Bestelbon, only the Akkoord.