EntityBase::toUrl() falls back to 'edit-form' as default $rel

EntityBase::toUrl() now supports entity types that do not provide a canonical link template but do provide an edit form link template. In this case, the edit form template is used when there is no $rel parameter. This allows modules that generically interact with entities of multiple types to call $entity->toUrl() and have it work as expected.

Entity types without a canonical link template and without a collection link template can now use the standard delete form. The cancel link will lead to the edit form of the entity.

No change for entity types with a canonical link template

$canonical = $entity->toUrl();

is the same as

$canonical = $entity->toUrl('canonical');

Entity types without a canonical link template

Before

$edit_form = $entity->toUrl();

This would throw a "No link template 'canonical' found" exception.

After

$edit_form = $entity->toUrl();

This is now the same as $edit_form = $entity->toUrl('edit-form');.