Add support for deleting items in the workspace

Fixes #559 

@karenc I tested this with the `delete-content` branch of cnx-authoring, but I always get a `401: Unauthorized` error.  Not sure if this is an issue with cnx-authoring or my setup.

Also, the `development.ini` needs to have `DELETE` added to its access control allow methods:
`cors.access_control_allow_methods = GET, OPTIONS, PUT, POST, DELETE`