Update documentation to reflect move to Alioth

While I have been updating the web pages for some of the stuff that was moved, there are still places that point to Alioth. Everything should be updated to point to GitLab.com, unless it is a historic reference worth keeping.

The same goes for manual pages, README's and similar files for the backends and frontends.

Here's a list based on eyeballing the results of git grep alioth[^-] on 2018-08-26:

• backends/AUTHORS
• backends/doc/releases.txt
• website/*.html contains link tags with href="mailto: pointing to Alioth
• website/sane2/index.html mentions cvs.alioth.debian.org!
• website/source.html mentions a link to showfiles
• website/unsupported/*.html references to Scanner Search Engine
• website/unsupported/canon-3200f.html references cvs.alioth.debian.org
• website/unsupported/hp-scanjet-4570c.html references bug report on Alioth