Some updates to the schemas

* alt on an image can be null
* cps_id is a string and must be unique
* format can be TextualFormat or VideoFormat
* highlight is a boolean field
* image can be null
Use cps_id as the _id for articles
Wire up on_insert_articles
Add a query for articles to the Juicer resource
Add fetch_articles task to dump Juicer articles into db