This is a straight port of the User Activity widget on top of the new

BaseAdminWidget Python class and RB.Admin.Widget/WidgetView

JavaScript classes and our modern CSS component naming conventions. The

new version works as the old one did, but behaves better when resizing

and when the layout changes.
A future change will rewrite this widget entirely, changing how we both

compute and represent the data in order to remove confusion around what

the calculated numbers actually mean. Right now, "Active" really means

"1-6 days," "7 days" means "7-29 days," "30 days" means "30-59 days,"

etc., while "90 days" means "90+ days". We don't communicate this, but

the JavaScript and Python implementations now at least document this.

Tested that the new widget provides the same information as the old one.
Tested that resizing works correctly.