`nbconvert` not installed in deepnote

[bernhardpg]

Running a notebook in Deepnote that requires Grader from manipulation.exercises.grader will throw an error saying that nbconvert is not installed (manipulation.exercises.grader imports `nbconvert here).

nbconvert is listed as an optional dependency, and the current Docker workflow installs manipulation[all], so nbconvert should be installed to Deepnote.

For now we have implemented the following workaround:

Install nbconvert in the Init notebook that runs when the machine on Deepnote is started: notebook.