CHANGE: getBannedSites now returns directly the list of banned sites

Instead of getting all the sites then filtering out those that are not
in the mask, directly get from the DB the Banned sites. Now
AllowedInSiteMask + BannedSites != total sites

CHANGE: Don't check if the site is in the SiteMask already, just set
the status.
FIX: documentation of banSiteFromMask

Fixes #294.