Standardize content entity routes.

Same story as for #253.

As I wrote there:
> For content entity types we also need to rename the "view" route to "canonical", and remove the admin-form link from the entity annotation (it has been renamed to field_ui_base_route and is no longer suitable for pointing to a settings page).

Untested. Needs one last change, figuring out the proper tab names in links.task-entity-content.yml.twig
(same as route names, or route names + '_tab'?)