changed date display in lists to human readable time spans

Hi! Thanks for AntennaPod!

I prefer the date display with a human readable time span in lists, so here's a little patch for that.

The second patch will change the feed list to show the most recent episode's date instead of the feed's last update time (which is the same for all feeds, anyway).

These patches will require a translation update.

P.S.: The first patch used to contain an "a moment ago" time span, this I have removed in the last two commits.

Hope you like it.