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.

**Note**: this branch was branched from engine at a point in time where integration between foreman/develop and katello/engine was already yielding the following error:

        lib/middleware/catch_json_parse_errors.rb:9:in `call'

Before I made these changes, this error was produced by visiting ```/katello``` when logged in as an admin user.
The same error still happens but with a different error heading rendered to the page:

**Original error heading**:
        
         RestClient::ResourceNotFound in Katello::DashboardController#index

**New error heading**:

         NameError in Katello::DashboardController#index

                 uninitialized constant Katello::Dashboard::SubscriptionsTotalsWidget

The root cause still *appears* to be the same, but I can't be entirely sure since both the katello/engine branch and this topic branch yield errors.  This may be expected as others have reported seeing the same error. @thomasmckay *may* also know something about this error.