Add indication for closed or merged issuables in GFM

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
• Tests
  • 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)