Added scale-to-fill styling for AMP post image
Added scale-to-fill styling for AMP post image

The featured post images are distorted in the AMP template. This change adds `object-fit:cover` styling, and thus avoids distortion but allows images to scale.

Fixes #11560
ErisDS/Ghost,TryGhost/Ghost,janvt/Ghost,TryGhost/Ghost,TryGhost/Ghost,JohnONolan/Ghost,JohnONolan/Ghost,ErisDS/Ghost,e10/Ghost,ErisDS/Ghost,bitjson/Ghost,janvt/Ghost,JohnONolan/Ghost,bitjson/Ghost,e10/Ghost