Drupal 8: Add view_mode to block templates, add view mode to template suggestions.

yourtheme.theme

/**
 * Implements hook_preprocess_HOOK().
 */
function yourtheme_preprocess_block(&$variables) {
  $content = $variables['elements']['content'];
  if (isset($content['#block_content']) && $content['#block_content'] instanceof BlockContentInterface) {
    $variables['view_mode'] = $content['#view_mode'];
  }
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function yourtheme_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  $content = $variables['elements']['content'];
  if (isset($content['#block_content']) && $content['#block_content'] instanceof BlockContentInterface) {
    $suggestions[] = 'block__' . $content['#block_content']->bundle() . '__' . $content['#view_mode'];
  }
}