Issue #3618261: The admin screens of a translatable record lost their form, their tabs and their order

Fixes the three things [#3616524] broke on the admin screens, and five faults of the same kind that testing them turned up. Nine commits, each one thing.

The mechanism behind the first is worth naming: a canonical link template makes core's route provider build a route, and it builds one that renders the entity through its view builder, which every content entity type has by default. A subclass that defers to parent::getCanonicalRoute() and only then composes its own has written dead code. Two routes on one path both match, one is served, and which one is nobody's decision, so they have to be interchangeable: core's menu_link_content declares the same form on both for exactly that reason.

Everything is tested, and every new assertion was run against the unfixed code and seen to fail. One catch worth repeating: $entity->toLink() asks for the canonical template and throws without one, so a breadcrumb built on it would have been an exception on every site not running content_translation. Only the kernel test saw that, because its module list has none.

Verified end to end against a running multilingual site, including that the owning reference still round-trips on save now that its widget is inaccessible rather than removed.

Drag-and-drop on the weighted lists that lack it, and the hand-built selects in the custom forms, are follow-ups.