`exporter_map` is deprecated, so let's use the list of exporters fetched
from the installed entrypoints.
There's a supposed attribute `export_from_notebook` that should be set
to a friendly string name if the exporter should be exposed in the
front-end. However, the exporters defined in `nbconvert` don't have it
set, so I haven't used it to determine inclusion in the list. Instead,
I've used the entrypoint name as the friendly name, which looks like it
was the intention from the way they are named.
I ran the unit tests and tried starting up the notebook server and
accessing the API endpoint to verify the JSON looked correct.
Min RK
Sally Wilsak
Steven Silvester