404-page: link to issue tracker
When a user view a 404-page, it might be because a mistake we did. We could be missing a redirect somewhere, for instance.
It's useful to know about these, so let's inform the user where they can report this.
But, as said in the last commit, encoding the template text into the URL like that is probably not a good idea in the long run. Perhaps we should use an issue-template for this instead?
Thoughts?