Preload card normal images for the feedback area

[scarletcs]

Currently the feedback area can have a noticeable period during which its height flickers becauase the Normal image needs to load in.

I'm addressing this in two ways:

• Feedback images now have a decent height and width configured.
• We preload the Normal image while you're busy working out the current art, reducing delays to nil.

This may just result in a brief flicker of the card image changing over, rather than a lengthy delay.

I also made these changes:

• Created getCardImages(), which extracts all the image_uris objects from a card. This is because we now need to do this complex process in more than one location.
• Extracted pickRandomItem(). It's only done in one location, but it helps describe what's actually happening in the code.
• Revised CardArt's imageUri computed property to use those two things.

I tried to add a little PreloadImage element to make preloading slightly more straightforward (and make it obvious that's what is happening) but for some reason, its :load event never emits. Maybe I did something obviously wrong that I completely missed. I added it in 7474537 during which it was immediately experiencing this error, and deleted it in 04d5e83.

[crookedneighbor]

Do we need test files for the new util files?

[crookedneighbor]

The vh class is usually used to hide the element, but make it accessible to screenreaders. Should the preload class just include the necessary stuff to hide the image so we're not using the vh class for multiple things?

[scarletcs]

vh also just makes it visually hidden, which is the behaviour we need. The preload class would just be duplicating it. It's already marked not for screen readers via alt="".