#508 (new feature) - Add ability to limit content placeholders to
particular sites

Requires following SQL update (site_manager module):

ALTER TABLE `site_sectionContent` 
ADD COLUMN `restrictBySite` TINYINT(1) UNSIGNED NOT NULL DEFAULT 0 AFTER
`allowInheritance`;

CREATE TABLE `site_sectionContentSite` (
  `sectionContentId` INT UNSIGNED NOT NULL,
  `siteId` INT UNSIGNED NOT NULL,
  PRIMARY KEY (`sectionContentId`, `siteId`),
  INDEX `sectionContentId_idx` (`sectionContentId` ASC), 
  INDEX `siteId_idx` (`siteId` ASC));