Sending metrics

In the admin page under "daily metrics", include a `Digests sent` column (the number of digests sent on a given day). Also include a "Global metrics" section that includes the number of subscribed users (i.e. people who are supposed to be receiving digests).

Start at the admin page's "daily metrics" section and trace how its existing metrics are assembled. Add the daily "Digests sent" count and a "Global metrics" section showing subscribed users; the work is done when both values appear correctly in the admin page for representative days.

Written by the indexing model from the issue text.