Make backends download from archived_backends after a few failed attempts

[tegonzalo]

As title says. If I download fails after a few attempts (3?) because the original site is down or the certificate is expired or something like that, then use the archived vesion in our archive_backends repo. There are two issues to be aware

• We need to make sure that the licenses of the backends allow for us to host the tarball elsewhere
• The maximum size of files in github is 100 MB (AFAIK). The tarballs of some backends are much larger than that (I'm looking at you, darksusy), so we might not be able to the same for those or we need to split them in parts.