Make footer sticky

Hi there!

I was about to suggest adding a footer partial, then I pulled the latest update to `main` and found you already added one. Perfect timing, and thanks!

I'd like to suggest making the footer 'sticky', specifically with the behaviour described and demonstrated [here](https://css-tricks.com/a-clever-sticky-footer-technique/), so it sticks to the bottom of the viewport *only* when there isn't enough page content to push it down, but *does* scroll down out of view rather than overlap page contents.