Add messaging for closed issue references in notes

Adds a new Banzai filter to give notification on links to issues that have been closed. This operates over the markdown after it has been created/pulled from the cache, since the state of the issue may change between being cached & being displayed.

Screenshot:

Fixes #1369 (closed)

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together