Simplify logic for registering new aliases, so that when a site is installed with a positive ID, the aliases for that site get registered in Piwik against the specified ID.  Prevent duplicate aliases in site_alias table.  Allow legacy sites which were installed before hosting_piwik was enabled to override their Piwik site ID during node edit, if desired.

Signed-off-by: Shai Sachs <ssachs@ngpvan.com>