Show discussions on expanded code lines on the Changes page

Right now it's possible to leave a comment on an expanded code line (i.e. not changed by the MR itself) in a merge request.

These comments won't be visible if you refresh the page because these lines are not displayed after a page refresh. Even if you expand the affected lines you won't see the comment. It will now only be visible on the Overview page.

We could handle that by doing one of these things:

1. Show a clickable indicator that the hidden lines have more comments that are not visible at the moment. When clicked it should expand the lines and show the discussions.
2. Render the affected lines on page load and show the discussions. This is not optimal performance-wise because the discussions will now affect diffs rendering (which is cached and that will invalidate the cache).
3. Something else...