Fix Flickr POTD

The API key is either expired or revoked by Flickr. It seems that Flickr really wants to restrict the API rates.

Solution 1 is to recreate an API key. This is easy. But we don't know when the key will be revoked again... For LTS distributions, it is almost impossible to update the API key...

Solution 2 is to parse HTML. But I found it is too complicated to get a full HD size image.

Solution 3 is store the API key in kde.org server, users will first fetch the key, then fetch the picture from Flickr API. In this way we can ensure a Flickr key can be updated anytime when it is found expired or revoked.