Init default content types site setting

Add a sites_content site setting that lets a site declare which entity types/bundles it hosts by default. New content of an opted-in type is preselected for the site, and with canonical_by_weight the lowest-weight opted-in site becomes canonical.

Refactor the inline initial-selection logic in the SitesSelection element into an InitialSitesSelection event resolved by priority-ordered subscribers (form state/user input, current site, content-type setting, current user fallback). Add unit coverage for the resolution order and the settings-form normalization.

Closes #3605686