Make Lightbox images fill the screen again

[paperboyo]

Hello,

To cut long story short, images in the Lightbox should always fill the screen.

Long story:
The lack of this obvious functionality was discussed here, where the only argument for not having it was to punish readers, so that the users would learn to use higher quality images. This problem is mostly gone/solved because of mediaservice. There are plenty of arguments for having this feature, too many for me to list them unprompted :)

Also, since we've used imgIX, images in Lightbox actually were filling it. The lack of fit=max meant we were requesting them upsized from imgIX. That was crazy for all sorts of reasons, but meant that this particular feature was there. It changed in #12079. So we are back to this (e.g. for all old galleries):

Let's get rid of that!

Regards
Mateusz

[paperboyo]

Test case: https://www.theguardian.com/commentisfree/2018/oct/15/president-trump-new-painting-white-house-republican#img-2