Use granular slug for internal channels

[eessex]

The fairs channel is making a number of articles with the same title 'about the fair' -- article date is attached to the slug, but there were a few made on the same date that now seem to have the same slug:
https://www.artsy.net/article/artsy-fairs-about-the-fair-01-11-18

We should do a check for uniqueness on slugs, or use a more granular date field to avoid this.

[kanaabe]

Hmm interesting. I wonder what the Fairs channel is doing differently that the timestamps don't work. Bc we do include more granular dates when we detect duplicate slugs which is supposed to avoid this exact problem. ie: https://github.com/artsy/positron/blob/master/api/apps/articles/model/save.coffee#L66-L67

[eessex]

Very strange -- yes I thought we were doing this already, for some reason two articles on the Fairs channel had the exact same slug

[eessex]

This article in writer goes to another in force on preview: https://writer.artsy.net/articles/5a5686c49953ea00330268db/edit