Add more validations for *SnapshotContentModels

[vojtechjelinek]

Some of the *SnapshotContentModels we have in out datastore are not valid. Some of the snapshot models contain fields that no longer exist in their parent model and that will cause an exception when we try to revert the parent model to some older version.

We should add more checks into the prod validation jobs for the *SnapshotContentModels, these checks would run the populate() method on the parent model a try to populate it with the data that are inside the snapshot in the content field, that way we can find out if we can revert to all of the previous versions.