References https://github.com/TryGhost/Ghost/pull/4373
* Consistent line-height & padding cross-browser
* Remove drag handle from textarea
* Partially clean up the CSS to more closely match the new standard
closes#4368, fixes#1240 (spellcheck), fixes#4974 & fixes#4983 (caret positioning bugs)
- Drop CodeMirror in favour of a plain text area
- Use rangyinputs to handle selections cross-browser
- Create an API for interacting with the textarea
- Replace marker manager with a much simpler image manager
- Reimplement shortcuts, including some bug fixes