Add subscription totals portlet to the dashboard

This "portlet" (aka widget) on the dashboard implements user story
"As a dashboard user, I want to see current subscription totals" in
SAM backlog sprint 10.

This portlet adds the following functionality:

- It links to the subscriptions search page, where the results shown
  are filtered by date range using elasticsearch.
- The counts shown are calculated from new methods on the Pool class
  in the katello app's models.
- I add a helper method to the DashboardHelper that permits a portlet
  or widget to exclude the quantity dropdown from displaying in cases
  such as this where quantity doesn't make sense.