Unsplash POTD: filter out banner image and pick random photo from latest

Summary:
Have been using this Unsplash POTD for several days and found the wallpaper doesn't change by day.

Then I found two problem in the code:

1. The HTML page contains a banner image, which almost never change. It is fetched first with previous regular expression.
2. The photos on the page doesn't update everyday. Sometimes they update a bunch of photos at one time.

So I changed it to global match and pick a random one from latest photos. Looks more like picture of the day.

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22451