Don't ignore private niteoweb repos in linkchecker

[zupo]

We run make linkchecker on every commit. This checks that links in our Handbook are valid.

However, some of these links are to private repositories and we have to skip the checking since the script does not have access to them.

It would be nice if it had though! So figure out how to have read-only access to those niteoweb repositories that we are linking to, and remove them from the ignore list on https://github.com/niteoweb/handbook/blob/master/Makefile#L17.