MergeRequestController: Rendering mentioned_issues_links and closing_issues_links is slow

From gitlab-com/gl-infra/production#895 (comment 183674697), rendering issue links takes about 30% of https://gitlab.com/gitlab-org/gitlab-development-kit/merge_requests/704.json?serializer=widget:

Part of the slowness comes from having to do a SQL query to find which nodes are accessible to the user and run this through the redactor filter:

If we wanted to cache this, we'd need to cache this content by user since every user has different visible nodes.

For now, we might just want to consider omitting this from real-time updates to save the 30% load time.