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) 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

### Screenshot

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/3d0a3213fde38844a681a826da18139b/image.png)

See merge request !810