Add option to download article lists via a PetScan or PagePile ID

[ragesoss]

PetScan (especially) and PagePile are the main tools used in the Wikimedia community to collect sets of articles for analysis. It will be useful to take an ID or URL with the ID from either of these services and generate an article title CSV from it.

The Dashboard has backend code for getting articles from both of these services (petscan_api.rb and pagepile_api.rb).