Change inventory relationship on medication to be async

[jkleinsc]

As part of addressing HospitalRun/hospitalrun-server#5, we need to change the inventoryItem relationship on the medication model to be async. Also, the medication record will need to have an additional field added to track the name of the inventory item associated with the medication record.
Once we do so, users who need access to medication records but do not need access to inventory records will be able to view medication records without the need to access inventory records.