Proxy images instead of linking to the repo's URL

[ruphy]

Some code hosting platforms (e.g. invent.kde.org) are not meant to serve images. Our current backend is more strict than it should, requiring that images are inside the repository, instead of accepting any URL as valid (as per standard).

Desired behiavor: We should accept any URL and allow software vendors to use any URL for their images, in the case they have a CDN or prefer to host their screenshots or other material on a different service.