What does this MR do?
This MR continues the work from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5993.
Adds a new Banzai filter to give indication on links to issuables that have been closed or merged. This operates over the markdown after it has been created/pulled from the cache, since the state of the issuable may change between being cached & being displayed.
Are there points in the code the reviewer needs to double check?
Screenshots (if relevant)
Does this MR meet the acceptance criteria?
Changelog entry added [ ] Documentation created/updated [ ] API support added
Added for this feature/bug
All builds are passing
Conform by the merge request performance guides
Conform by the style guides
Branch has no merge conflicts with
master(if it does - rebase it please)
Squashed related commits together
What are the relevant issue numbers?
Closes #1369 (closed)