First reported:
  https://gitlab.com/gitlab-org/gitlab-ce/issues/60143

When the page slug is "javascript:" and we attempt to link to a relative
path (using `.` or `..`) the code will concatenate the slug and the uri.
This MR adds a guard to that concat step that will return `nil` if the
incoming slug matches against any of the "unsafe" slug regexes;
currently this is only for the slug "javascript:" but can be extended if
needed. Manually tested against a non-exhaustive list from OWASP of
common javascript XSS exploits that have to to with mangling the
"javascript:" method, and all are caught by this change or by existing
code that ingests the user-specified slug.