Adding user validation during `create_inactive_user` and `create_user`

One point of note is that ~42 thousand users currently have data in the gravatar field that will fail validation.  This will need to be handled before/during deployment, likely via a `Users.objects.filter(gravatar='none').update(gravatar='')`.  This will handle the vast majority of cases.  
We may also need a `Users.objects.filter(gravatar=None).update(gravatar='')`