Move + and - of diffs inside the gutter

Problem

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4789 implements diff viewing in the Web IDE. The + and - are displayed inside the gutter there, which actually separates markup from content better than our way of styling in normal diffs throughout the application.

Solution

Let's improve that by moving the - and + inside the gutter similar to the web ide and make things consistent at the same time