Admin Analytics: Include current number of search notebooks

Created by: mike-r-mclaughlin

Requested by: https://github.com/sourcegraph/accounts/issues/3

Feature request description

On the Notebooks page, there is currently only "Notebook views" and "Notebooks created". Please add a new metric for "total notebooks" to allow the admin to track how many notebooks are in the system at a given time (i.e., to allow them to account for deleted notebooks).

Is your feature request related to a problem? If so, please describe.

It's difficult to know how many Notebooks are in Sourcegraph. Notebooks created tells you how many have been created but there is no way to know if those notebooks were later deleted.