Support commenting on a diff in side-by-side view
What does this MR do?
This MR adds support for commenting on a diff in side-by-side (aka parallel) view. It also fixes a JavaScript bug (see !779 (closed)) when the comment button is clicked on a line that already has a comment.
There is an existing bug where the comment count is not updated when a new comment is added. I'll send a MR for that later.
Why was this MR needed?
Commenting only worked in "inline" mode. Often the side-by-side view is more conducive to writing comments.
What are the relevant issue numbers?
Closes https://github.com/gitlabhq/gitlabhq/issues/9283