Custom Boost progress bar

A server I'm an admin in recently enabled the Boost progress bar, which got me thinking. I reckon it'd be quite cool if you could swap out the default 🎉, perhaps even allowing server managers to customise the gradient as well. Although, I would be happy enough with the ability to customise the emoji, especially if it allowed custom emojis rather than just the built-in ones.

To give a sense of where I'm coming from, the server I reference above is aviation-based, so ideally we'd switch the emoji to a plane and gradient to something bluer for the sky.

Even if this did just allow the emoji to be swapped out for a built-in one, I think it'd be a nice little addition.