Entity types can specify plural labels now

Entity types can now provide plural labels, which allows for better generic UIs and interface text.

It is strongly recommended that each entity type provides the following annotation keys:

 *   label_singular = @Translation("content item"),
 *   label_plural = @Translation("content items"),
 *   label_count = @PluralTranslation(
 *     singular = "@count content item",
 *     plural = "@count content items",
 *   ),

Without them, Drupal will default to more generic versions ("@label entities", "@count @label entity" / "@count @label entities").