Fallback campaign mechanism implemented

In order to assign "fallback" attribute to certain advertising campaign, network operator may do this by going to "Edit campaign"->"Use as fallback" checkbox. Campaigns' creative will be displayed for the whole network at all advertising sites in case their advertising spaces stay unoccupied by commercial/partner ads. Fallback campaigns are automatically assigned the lowest priority level so that they won't get in conflict with commercial ads. Also, "Fallback" attribute automatically abolishes all targeting settings of advertising campaign.