src/themes/dspace: rework banner image

For some reason our version of this image is twice the size of the
original (~860KiB vs 1.6MiB). To make matters worse, the dimensions
of the image are 4777x3166 pixels, but we display it at 2000 pixels
wide.

This regenerates the images based on the original and adjusts the
dimensions to be more appropriate. Generated using ImageMagick 7
on Linux:

$ magick pexels-inspiredimages-132477.jpg -resize 2000x src/themes/dspace/assets/images/banner.jpg
$ magick pexels-inspiredimages-132477.jpg -resize 2000x src/themes/dspace/assets/images/banner.webp
$ magick pexels-inspiredimages-132477.jpg -resize 1200x src/themes/dspace/assets/images/banner-half.jpg
$ magick pexels-inspiredimages-132477.jpg -resize 1200x src/themes/dspace/assets/images/banner-half.webp

See: https://www.pexels.com/photo/clear-water-drops-132477/

Author: alanorth