[#1936] Avoid private and deleted datasets on stats plugin

The following queries have been udpated:

* Top groups: avoid deleted groups

* Most edited datasets: avoid deleted and private datasets

* Top rated packages: avoid deleted and private datasets (I don't think
  this works anyway)

* Top package owners: don't include deleted or private datasets in the
  counts

Tests updated.
Merge pull request #2115 from ckan/1936-stats-plugin-active-datasets-2

[#1936] Avoid private and deleted datasets on stats plugin