Issue #3619199: Read the front page and the front-end content from this site

Five configuration objects of a module belonging to one site, published nowhere, were read by this one. A site installing this module has none of them and no way to learn that it is expected to.

Which page is the front page is the site's own answer and core keeps it in system.site. It is read from there now, in the three places that used to read it out of the foreign object, and the response finally declares that it depends on it: both the metatags and the locales are built from the front page, and neither carried a cache tag for it.

The other four hold content the endpoints hand the front end - the contact block, the footer, the award icons and the interface strings. Which objects those are is a setting now, one name per role, and an endpoint whose object is not named answers with the empty structure rather than with a missing key or an error. The cache tags follow the naming as well as the object.

An update writes the names a running site already had, for the objects that exist on it, so nothing moves for a site that has been running this module.

The service is reached through a property set after construction: the constructors of the base resource and of the helper are inherited by modules of the sites, and one more argument on either is a fatal error there.

Closes #3619199