This branch add an "admin action" to the site
https://wl.widelands.org/admin/wlimages/image/

The "action" changes the path form the "image"value from an absolute path to a
relative path in a convenient way. See
https://code.launchpad.net/~franku/widelands-website/handle_big_images/+merge/247235/comments/628869
for a reason why this is needed.

See also:
1. https://code.launchpad.net/~franku/widelands-website/correct_image_paths/+merge/254990
2. https://code.launchpad.net/~franku/widelands-website/handle_great_images/+merge/254988