Some special characters in headings do not render like commonmark

Some special characters in headings do not render like GitLab's markdown. For example, / is completely stripped out in GitLab's markdown, whereas it's replaced by - in Kramdown.

https://gitlab.com/gitlab-org/gitlab-ee/blob/71ec5152e36fef1d81a6e75a6d7e69e2c554494d/doc/workflow/notifications.md#issue-epics-merge-request-events VS https://docs.gitlab.com/ee/workflow/notifications.html#issue--epics--merge-request-events

We found out in gitlab-org/gitlab!10441 (comment 154511060).

We might be able to replicate the same behavior in the gitlab-kramdown gem, possibly in https://gitlab.com/gitlab-org/gitlab_kramdown/-/blob/master/lib/gitlab_kramdown/parser/escape.rb.