Lazy load merge request description
Right now merge request description is rendered no matter what merge request page is rendered for the client. For example if you open merge request pipelines page using a direct link you'll still get MR description rendered on the page.
We should extract this description into a separate template and render it:
- With the page if the page is Overview
- Using an API if the page is not Overview
This should improve the performance for the merge requests that have a very big description.