RFE: Add sorting option based on article length

**Is your feature request related to a problem? Please describe.**
Yes. Wallabag for Android can sort articles by chronological or reverse-chronological save order, but it doesn't have the option to sort based on article length. By sorting the articles by length, I can listen to a burst of short articles during a break, or long ones when I have a more lengthy and more focused period. Pocket had this option, and it's one of the few features I'm missing since my migration to Wallabag, which is fantastic.

**Describe the solution you'd like**
Change the sort button to a dropdown that lets the user pick sorting by save time or article length. Fortunately, the article length is already available in Wallabag, so the RFE effort would mostly be around exposing the capability inside the UI.

**Describe alternatives you've considered**
I haven't been able to think of alternatives to get around this.

Thank you for considering this, and please keep up the great work!