Surveys should have a created date

Makes sense to keep track of creation dates. 
If there's a slug but no creation date, then will we skip the part where we assign `this.created` a value? That seems unlikely, but the flow isn't clear because of that. If we're making the assumption that any survey with a slug also has the `created` field already, then can you mention that in the comments? Otherwise, let's move the `this.created` assignment up to the top of the function.