Set flex-direction for thumbnails to avoid overflow issues

`flex-direction:column` causes the flex-child `a` to grow it's height
and sets its width to 'auto' which, like a block-element, makes it
occupy the "available" space.

Bug: T200518
Change-Id: I5af9d082e77dd4ca2d92460824977f085011e622

resources/skins.minerva.content.styles/hacks.less

@@ -184,6 +184,8 @@ FIXME: Review all of these hacks to see if they still apply.
 			justify-content: center;
 			flex-wrap: wrap;
 			align-content: flex-start;
+			// avoid image child overflowing the container (T200518)
+			flex-direction: column;
 
 			> .thumbcaption {
 				.flex( 1, 0, 100% );