Port to Plone 5 / 6, exclusively.

[Rudd-O]

Things that have happened here:

• The canonical URL functionality has been removed because Plone 6 already generates those URLs.
• Most og: meta tags have been removed since Plone 6 already supports them by default.
• Archetypes dependency has been removed.
• CSS / JS registry ported to Plone 5/6 app registry and bundles. I still don't know why the event.log says that it failed to cook the javascripts when the add-on is installed / upgraded.

Tests have been ported, but some tests died here.

Translations have not been updated, although strings were only removed, not modified or added.

[Rudd-O]

Oh wow that is truly awesome — and you rock!