Add a check for unwanted external links in HTML output

[PeterFeicht]

There should be a check to make sure there are no unwanted links to the online version in the output HTML (so things like the broken release v20180527 can't happen again).

The Makefile already has a check target, which would probably be the place to add this to.

[PeterFeicht]

We could also check relative links to make sure they point to existing files.