Content translation operation is only available for nodes, not other entity types such as custom blocks

Problem/Motivation

Currently the translate operation link is available only for nodes. See:

function content_translation_entity_operation_alter(array &$operations, \Drupal\Core\Entity\EntityInterface $entity) {
  // @todo Use an access permission.
  if ($entity instanceof NodeInterface && $entity->isTranslatable()) {
    $operations['translate'] = array(
      'title' => t('Translate'),
      'url' => $entity->urlInfo('drupal:content-translation-overview'),
    );
  }
}

Proposed resolution

1. Implement hook_entity_operation() instead.
2. Instead of relying on NodeInterface and $entity->isTranslatable(), use content_translation_translate_access() and look for the link template.
3. Add tests.

Remaining tasks

Commit.

User interface changes

Translate operation will show among entity operations, such as this operation list for blocks:

API changes

None.