Add indication for closed or merged issuables in GFM

Merged username-removed-378947 requested to merge adam-finish-5993-closed-issuable into master

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?

What are the relevant issue numbers?

Closes #1369 (closed)