- Adds gh-view-title component to implement mobile menu button for titles on any page
- Refactors the `content-cover` out into the application template
- Fix various z-index issues with content-cover and gh-alert
- Move `.settings-menu-expanded` application view state from body to `.gh-viewport`
- Unify nav menu / mobile menu actions and code
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