Oh sweet, I learned about a new CSS feature with good-enough support!
This lets you use CSS transitions for an element as it enters the page,
or becomes visible.
Firefox only has partial support for this feature rn, but its partial
support covers our case, I tested to make sure! (Specifically, it
doesn't handle transitioning from `display: none` yet, which isn't what
we're doing.)
No more of this loading everything into `application.css`! I'm
arbitrarily starting here because that's where I've been playing
lately, but this is part of a larger effort to move toward a more
straightforward CSS architecture (and away from Sass even?)
