Handle user updates for closed issues

Problem to solve

When viewing an issue with a lot of comments, it isn't obvious from the bottom of the issue that the issue is closed. Additionally, users receive no feedback when commenting on a closed issue.

Further details

The distribution team occasionally gets comments on closed issues from users. It is difficult to track this situation and ensure a response is received.

Proposal

Add a visual warning to the comment box on closed issues.

Adding the issue status to the right sidebar may also be useful.

As an alternative, prevent user updates to closed issues, require an explicit reopen before commenting. I think often we want users to open new issues anyway, so this might not be preferred.

What does success look like, and how can we measure that?

Fewer user comments on closed issues

Links / references

omnibus-gitlab#2204 (comment 27271171) is an example -- There is the timeline entry of the issue being closed, but this is all gray text, and further comments will push it out of view.