Implement Notebook Diff usage Metrics

Why

We need data to better assess usage of Jupyter Notebooks and the diffs we are working on. This will allow us to answer hypothesis such as "Does having rendered notebooks diffs increase the likelihood of a notebook being reviewed?" or "Should we have the raw or the rendered viewer as the main one?"

Work to do

backend

• New comment on diff with notebook (all, commit, mr)
• Unique users that created a comment on a Notebook (all, commit, mr)