Rework image thumbnails

[multimeric]

When you run a command like !card Ice-Fang Coatl, it embeds a thumbnail image, which is a good size for a bot output:

However, when you click on that image, it becomes evident that it is very low resolution, and is basically unreadable:

(this an exact copy of the image that is embedded, at the same size)

Therefore you need to click the magnifying glass to embed a full-resolution image, but the original thumbnail remains.

I propose two changes:

1. We embed a higher-res thumbnail, which is displayed at the same size as before, but when clicked on will be legible. I know this is possible because this is exactly how the Scryfall bot works. e.g. the output from [[Ice-Fang Coatl]]
2. When the magnifying glass is clicked, we hide the thumbnail from the original message, because it clutters it up with a duplicate of the same content

[multimeric]

This is mostly fixed by #159.