`isDeleted()` from `deletedDate`

[WrathfulSpatula]

We currently have an isDeleted boolean on the User schema, and now similarly on the Submission schema, but we have no record of when deletion occurred, at all. This could be a critical piece of information, for database administration.

We should convert these isDeleted boolean fields to deletedDate fields with a model logic wrapper, isDeleted(), which returns true if any deletedDate is present and false is the deletedDate is null. I can handle this for the User model, but please do this for the Submission schema you're working on now, @vprusso.

[vprusso]

Sounds like a plan! I'll see what you are able to whip up for on the user side of things and apply it accordingly to the submission portion. Thanks!