Fix mount action serializers, remove unmount action models

Goal of this MR:

• fix the serializers for the device- and platform mount actions as well as for the configurations
• remove unmount action models and properties
• remove and fix all code that uses the old unmount actions
• fix actions page and timeline (at least for the mount actions)
• fix existing unmount form
• fix loading of the "Dates defined by actions" dropdown after mounting / unmounting an item