Augment `seo` data on entries/terms to include cascaded site and section defaults

[jesseleite]

It would be nice if users could get an entry's/term's full seo cascade with site and section defaults (in templates, via api, in php, etc.) by calling $entry->seo or $term->seo.

Right now, this is only possible by building the cascade yourself as described in the workarounds here, but this should probably be default functionality.

Note: If someone needs an entry's/term's seo data without the cascade, they could just call $entry->raw('seo').