Always read diff_view setting from the cookie

Prior, when the user had their view set to "parallel" and then visited a merge request's changes tab without passing the view parameter via query string, the view would be parallel but the Notes class was always instantiated with the default value from diff_view ("inline"), resulting in broken markup when the form to add a line note was dynamically inserted.

The cookie is set whenever the view is changed, so this value should always be up-to-date.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14557 and https://gitlab.com/gitlab-org/gitlab-ce/issues/15285