Dashboard statistics should use 'period' as the date limiter

The statistics module on the dashboard was using the incorrect query parameter
'stop'. This meant that the module was loading all tickets from the report start
date to the present day. This commit fixes this by using the 'period' parameter
instead.
Merge pull request #1345 from mattkirman/statistics-use-period

Dashboard statistics should use 'period' as the date limiter

Reviewed-By: Jared Hancock <jared@osticket.com>