Omit issues links in merge request entity API response
The merge request widget has a section that includes which issues may be closed or mentioned based on the merge request description. The problem is that rendering and redacting Markdown can be expensive, especially since the browser polls for the data every 10 seconds.
Since these links don't change much and are just nice to have, we only load them on first page load. The frontend will use the existing data if the data doesn't appear on subsequent requests.
This saves about 30% of the rendering time of this endpoint, which adds
up to significant savings considering that
MergeRequestsController#show.json is called over a million times a day