fixes#5136
- wrap notification fetch with a user role check to remove console error
- move author transition down to local route for users/user so that there's no infinite loop
- replace all store calls to fetch the current user with the session user instead
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