Get article "From the DDR" links from Elasticsearch

Data for the "From the Densho Digital Repository` section of article pages is retrieved by API calls to the DDR API during article page requests. `encyc-front` caches these results but they're still coming from `ddr-public` every 15min or so.

The code for getting the data from `ddr-public` should be moved from `encyc-front` to `encyc-core`. The API calls could occur during the article publish process, but might be better as a separate class, analogous to `Sources`.

This would result in fewer connections between `encyc-front` and other parts of the infrastructure during page requests.