Adding a message about drop in dataset #s

[djbrooke]

The Metrics app is encountering some issues due to how INRA and Manitoba have configured their Dataverse installations. Instead of making the changes in the Metrics app itself, we are asking the installations to make updates to their configuration. While we wait for these installations to update, we are going to add a note on the Metrics app itself, primarily to explain the large drop in Dataset #s from month-to-month due to the outsize # of datasets in INRA.

This is my attempt at doing this - adding a subtitle element to the D3 chart.

[pdurbin]

This looks fine to me as of 74d5296. With a subset of data (Harvard, INRAE, and Manitoba), here's how it looks on my machine: