Support drag and drop notebook creation

[openjck]

Users should be able to create new notebooks by dragging and dropping files onto the homepage.

If an authenticated user drags and drops files...

a new notebook should be created which is associated with their account and which has all dropped files attached. The notebook should use a template which explains or demonstrates how to use the files.

If an anonymous user drags and drops files...

they should be redirected to the tryit page with all files attached in a temporary fashion. They should be allowed to open the Manage Files modal to see the attached files and delete any as needed. The notebook that is shown should both explain how to use Iodide (as the tryit page does now) and explain or demonstrate how to use the files. If the user logs in or creates an account later in the same session, any changes they made to the notebook and any files that were dropped and note deleted should be saved and associated with their account.

We should also make the Manage Files modal usable on the tryit page generally, otherwise users may be confused as to why they can use it when they drag and drop files but not when they navigate to the tryit page directly.

Mockups

https://www.figma.com/file/xjDZZBXDjWzKJbh5EjtWgl/Drag-and-drop-onto-homepage?node-id=0%3A1