Random select hero svg image on page reload

[peterkos]

On every page reload, randomly select one of the three desk1.svg, desk2.svg, or desk3.svgs in assets/ for the hero section of the site.

I feel like this is fairly easy to do with an array, background-image (already implemented), and a bit of SASS.
For reference: https://blog.krowinski.com/sass-scss-selecting-a-random-element-from-an-array.html

[peterkos]

Looks like it needs to be done w/ JS as SASS's random vars are only generated once on-compile, not every page load.

[peterkos]

Blocked by #1033 as that's modifying a lot of hero CSS, and SVGs