Improve handling of `hreflang` and `canonical`

[aerni]

This PR adds some improvements around the handling and generation of the hreflang and canonical tags.

• If multi-site is disabled, the hreflang won't be generated.
• If an entry/term/taxonomy only has one localization, the hreflang won't be generated.
• If a page is not indexable hreflang, canonical, prev, and next won't be generated.
• Set the correct x-default in sitemaps when the origin of an entry/term isn't indexable.
• If canonical points to another URL, the hreflang won't be generated.
• Hide the Canonical URL section in the SEO tab if noindex is enabled.
• Validate that the seo_canonical_custom is an active_url.