Regression: "Back to top" button covers footer

[zackkrida]

The back to top button can cover the contents of Openverse's footer, like so:

This is a particularly egregious problem when we observe the dark mode footer, which moves controls for site color theme and localization to the right corner of the footer:

The best solution is likely to move the button in the DOM order and make it sticky positioned relative to a parent container, instead of fixed to the document.