Drupal 8: Add view mode suggestion for taxonomy terms.

/**
 * Implements hook_theme_suggestions_alter().
 */
function THEME_theme_suggestions_alter(&$suggestions, $variables, $hook) {
  switch ($hook) {
    case 'taxonomy_term':
      // Add a view mode suggestion for taxonomy terms.
      $suggestions[] = $hook . '__' . $variables['elements']['#view_mode'];
    break;
  }
}