Deleting a snapshot tied to a goal doesn't fully remove the snapshot from the goal

Issue Summary

This is reproducible.  Deleting a snapshot appears to delete, but we can still see the snapshot associated with the goal via the API, even after it is deleted.

Steps to Reproduce

1. Create a snapshot
2. Create a goal
3. Associate the snapshot with the goal and save it.
4. Through the UI, delete the snapshot
5. Using the API, do a GET on the goal ID created above

Expected Results

The previously associated and deleted snapshot ID should not appear in the data.

Actual Results

The deleted snapshot still exists in data for this Goal.  

Also - since there is invalid data associated with this Goal now, we are unable to perform a PATCH operation on this Goal.  Doing so will throw an error indicating that the snapshot ID is invalid.

Workaround

No known workaround