Move Astropy Learn to its own org

[ceb8]

In my role as Astropy Learn coordinator I am proposing that Astropy Learn is moved out of the Astropy Org and into its own Github Org.

The reasons are thus:

• Most important: The organizational structure of the Learn tutorials is being updated (for maintaiability and ease of adding new material), and as part of that each tutorial will get its own repo. This would clutter of the Astropy repo, and is a big reason for moving Learn to its own org
• This will also alow the seperation of Astropy and Learn permissions and github secrets, which will allow for better seperation of elevated privileges.