post tooltips: remove gap beneath thumbnail on post #xxx tooltips.

Set the thumbnail height to auto instead of 154px so that there's not a
big empty gap beneath thumbnails that aren't 150px high.

This requires setting dimensions on the <img> tag itself so that the
image height is known before loading, otherwise the tooltip will be
mispositioned after the image loads in.

We set the min-width and min-height instead of the <img> width and
height properties because our calculated dimensions are sometimes
off-by-one compared to the actual dimensions. I'm not sure how libvips
calculates the thumbnail dimensions, but this avoids forcing a slightly
wrong aspect ratio, which produces ugly resizing artifacts.

app/javascript/src/styles/specific/post_tooltips.scss

@@ -74,6 +74,7 @@ $tooltip-width: 164px * 3 - 10; // 3 thumbnails wide.
 
         article.post-preview {
           margin: 0 8px 0 0;
+          height: auto;
         }
       }