Repair broken links
This completes my check of external links. Dead links were fixed from Internet Archive unless better options were found.
- A Bugzilla bug was replaced with its successor on Gitlab since that one still is open.
- xcf specs has moved to gimp-web-devel
- Better section links for two Wikipedia pages.
- Use double quotes for the only ulink that used
' '
. The single quotes messed up opening the link from my terminal, it interpreted it as an address with a trailing quote for some reason.