Move source media files out of images and delete unused

We have a lot of files in static/images/ that aren't used, mainly because
the page that used them has been deleted without deleting the images or are
source material used to generate the images that are used.

Move the source material to `source-media`. Delete all unused files.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>