* Implement cells bin
* Temporarily keep cells source
* Send complete bin entries from session to clients when a cell gets removed
* Polish styles
* Hydrate bin entries on section deletion as well
* Set up markdown rendering, update theme.
* Improve focus and handle expanding for markdown cells
* Add keybindings for expanding/navigating cells
* Improve editor autofocus when navigating with shortcuts
* Add tests
* Render markdown on the client
* Don't render cell initial data and make a request instead