Add mount id for the availability endpoints

When editing a device in the frontend we still want to get the availability information (to see how far in the future the end date could be). However, it will still return that the device (or platform) will not be available as there is the action that the user wants to edit in the frontend in right that moment.

We want to add the mount id to the payload of the availability endpoints, so that we can ignore that single entry in the payload & still get the information about the other contraints in availability that we need to consider.