Add fallback urls for cards without a muId set

This is still a proposal that could need discussion.
A lot of cards have no multiverse id, so we are not able to download these card's pictures. This results in pictures of the same card from other sets being displayed, or no picture at all.
This commit adds a fallback url using set+name instead of muid to retrieve the card image, used automatically on crds with muid=0.
I only know a good HQ source for the pictures using set+name; any idea on a low-quality source?