Added Gallery option to display image dimensions

[RalphORama]

Adds an additional info panel (disabled by default) that shows the dimensions of the image you're viewing in gallery mode. This is useful on boards like /wg/ where one would want to quickly see the resolution of the image they're looking at.

The dimensions display under the filename and above the sauce links:

Additionally, when the option is disabled, the dimensions span is set to display: none so it doesn't add any extra spacing/padding and make the UI look weird.

This closes #198 (from 2014!)