Group growth V2

A big step forward for the extension; please see updated README. In brief: adds more stats, works with smart groups, shows mailable totals not (just) group membership size, interactive chart.

I'm putting this PR here to increase visibilty of the v2 branch (with apols to @JonGold)

Nb. this branch will be merge-ready once there's a way to configure which groups you want to monitor.