Linter for checking broken links
Follow up from https://gitlab.slack.com/archives/C0AKZRSQ5/p1529683855000667
We have a linter (allowed to fail) that checks for relative links and that's okay. But we need one that checks for broken links (404s); it's essential for complex MRs that involve more than one page. We can easily check one page with whatever tool we want, but for complex MRs, such as release posts, features.yml, comparison pages, etc, it's important to rely on the system to do it for us. This job should be triggered by any push and not to be allowed to fail.