Clearing outputs in frontend doesn't clear outputs in ydoc and on disk

[ellisonbg]

When a user clears the outputs for a cell or the whole notebook the view is updated, however:

• The outputs remain in the ydoc.
• The outputs remain on disk.

This means when you reopen a notebook later the cells are not empty.

[3coins]

@ellisonbg
While #135 has taken care of some of the backend code to clear outputs, the remaining issue seems to be that the front end never sends a signal to back-end to clear the outputs, so the outputs are never cleared in the ydoc, this cannot be handled by updating the shared model since the outputs are no longer stored there. We can handle this in one of 2 ways:

1. Add a new API to clear outputs and call this when the clear output action is performed
2. Send a clear_output kernel message (Not sure if this has a clear all outputs equivalent)

[ellisonbg]

This is fixed in main.