make max_extractable_spreadsheet_size a configurable setting

rather than being hard-coded
defaults to 10Mb
methods on model to get view and download counts for a resource

removed from view helper method, and instead added to module mixed into isa and assets
moved other stat modules into a new seek/stats namespace
added a Top of the Pops rake task to get top downloads and views
OPSK-87 - updated sunspot search field used for models#matching_data_files