Add height and width attributes to ballot cover images

[dfabulich]

This prevents layout shift on the ballot page.

Fixes #453

[dfabulich]

I'm not 100% sure this is the "best" way to do it, but I'm trying to fix #453 in a hurry, and this certainly appears to work on my dev machine.

In particular, this code will run Imager to get the height/width of all cover art every time the page loads. That seems inefficient. But I bet it's manageable, and we can improve performance with a cache in a subsequent PR if we find that the performance impact is too severe.

[coveralls]

coverage: 62.02% (+0.07%) from 61.946%
when pulling 9af07c0 on dfabulich:ballot-image-dimensions
into 3550103 on iftechfoundation:main.