The current Downloader class works pretty well, but is entirely redundant to $.ajax(). We should think about replacing the former with the latter, and maybe putting the Deferreds we get from ajax() into pg.ajax, a new list.

While we are at it, might as well replace it with mediawiki.api instead. This gives us

• token refresh
• centralised error handling
• fewer needed configuration options.

Do make sure to retains downloadsinprogress, used for aborts.