Handle autosave from multiple devices (GitHub session lock)

[taramk]

This doesn't have to be perfect, but if there's anything we can do to prevent an older version from saving over a newer one. Checking more often for updates and refreshing the page/showing a warning about conflicting changes? idk. Happy to discuss this.

When I saved a small update from my phone it seems like it overrode some desktop updates and/or those desktop updates didn't get autosaved at all because I can't find them in the commit history.

[taramk]

We've decided to do a GitHub session lock at least for now. So they can only be logged in to one device at a time. When they log into a new device, show a notification on the logged-out device. Tara to provide more definition for modal etc.

[taramk]

Modal:
https://www.figma.com/file/bGzEZo0NYHhWGJO95kwF3p/Treehouse-App?node-id=8%3A211&t=TTAcBgIYJHyni8io-1