* Move clear stats functions to Plausible.Purge

* Delete both native and imported stats when deleting a site

This commit moves the delete site function to the Plausible.Purge
module, and fixes a bug where deleted sites could leave dangling
imported stats.

* Clear sites.stats_start_date after clearing stats

This commit fixes a bug where resetting stats left an invalid state of
the stats_start_date field, used for GA imports, for example.