Sort articles by reading time

Now we can sort articles by "aggregation date". Will be nice to add an option to sort by reading time because sometimes you don't have too much time to read.., Related #492

Start by locating the existing article sorting code for "aggregation date" and inspect how its option is exposed in the Android app. Add reading time as another sorting choice, then verify that the article list orders entries correctly by reading time.

Written by the indexing model from the issue text.