Switched d-i boot screens to use the homeworld theme

Hey folks,

I've pushed changes into d-i just now so we'll use the lovely new
homeworld theme from Juliette for our boot screens, both for isolinux
and grub.

@Juliette: I've tweaked the size and positioning of the "debian 11"
logo a little from what I found in your git repo. I hope you're ok
with this! :-)

Reference screenshots at

  https://www.einval.com/~steve/tmp/homeworld/

so you can see what I've done. The original placement for the logo
clashed with the grub help text at the bottom, and I think looked a
little small and lost at the bottom of the isolinux screen.