Add an "unsorted" indicator to table columns

This came up in https://github.com/backdrop/backdrop-issues/issues/3906#issuecomment-908...

@laryn

Am I bikeshedding with this? Riffing on https://github.com/backdrop/backdrop-issues/issues/3906#issuecomment-745... I did a quick mock with the bars visually unsorted:

Using:

@klonos

I like that icon @laryn ...is it fontawesome, and if so, then which one is it? (it would have to be black or some dark gray color though, since the unsorted headers are light gray and we'd have accessibility issues)

@laryn

@klonos It's taken from the fontawesome icons you were using and then tweaked in Photoshop. I'm sure we can adjust colors on the SVG and then export as a PNG if we know which colors to use.

@klonos

@laryn I admire your creativity! ...you've made an icon that I find to be quite accurately denoting "unsorted"

Can you please provide a transparent 64px .png in #444b53, and another in #000000? I wanna "play" with it

@laryn

@klonos unsorted.zip