Article app (with Wallabag and Readeck support)

[aykevl]

First version of built-in Wallabag support, see #400.

There are a number of things that aren't quite there yet:

• The long-press menu on an entry doesn't actually do anything. This needs to be implemented.
• Related: starring/archiving/removing isn't implemented yet.
• There's no download progress, there should be some progress indicator.
• There's currently nothing that prevents two syncs from running at the same time (and messing with each other).
• All articles are always downloaded. Ideally, only new unread articles would be downloaded.
• The epubs downloaded from Wallabag aren't great, with a lot of fluff. We might want to create our own by converting them from HTML to custom epubs.

(This PR also includes #397, I can remove it but added it for my own convenience).