Improve site banner code

[Stefan4472]

• Using a jumbotron to display the image is problematic, because the image needs to be loaded via url(). Replace the jumbotron with a regular HTML element, so that we can use a regular image element.
• Replace the Javascript dynamic image loading with a media query

[Stefan4472]

Rolling this into #105 because Bootstrap 5 actually removed the Jumbotron.

[Stefan4472]

Investigate whether we can have a single image to use as a site banner for all screen sizes.

Probably, it would follow a 1.9x1 w/h ratio and be fairly high-res. On large screens, we would avoid stretching it out and instead blot-out the sides of the screen with a blurred color effect (or something).

This would allow us to make the banner image a pre-load.

[Stefan4472]

After further investigation, changing to a single image isn't worth it. The performance gains would be very minimal and I think the user experience would be worse. The current solution with two different-sized images is pretty nice!