WIP: Use created_at date instead of published_at to order articles

This MR fixes #42. This MR changes the data model!

For this first commit in the MR, it assumes the app is at first launch and there is no articles in the database. Further commits will fix this behaviour, to wipe the database before re-sync'ing with the server.

This MR means the whole collection needs to be re-downloaded. This can be an issue for large collections. It would be wise to wait for Wallabag to release version 2.4, which implements pagination and filters for the query.