closes https://github.com/TryGhost/Ghost/issues/9635
- CodeMirror expects default CSS box-sizing rather than `border-box` so it's scroll calculations were thrown off
- reset the `box-sizing` properties on CM elements
- remove now unnecessary `.CodeMirror-scroll` patches that we'd put in place to cover broken scroll behaviour
- move all non-layout-specific CM styles into a `components/codemirror.css` file