Implement SiteListFileCache and rebuild script

Provides file-based cache of the SitesStore data,
using a static json file dump of the data from the
SiteSQLStore.

Includes a maintenance script to rebuild the sites cache.

Bug: 56602
Bug: 45532
Change-Id: Iaee4c1f9fb5d54efe01975f733ebd5c339ac106f