No more CodeMirror

closes #4368, fixes #1240 (spellcheck)

- 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
- Rename components include editor-route-base
Textarea Editor CSS Improvements

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