enhanced plugin support (#634)

- refactored admin controller and templates
- plugin support for entity actions
- changed column mail to email in customer table
- refactored theme events

src/DataFixtures/InvoiceFixtures.php

@@ -21,6 +21,8 @@ use Faker\Generator;
  *
  * Execute this command to load the data:
  * $ php bin/console doctrine:fixtures:load
+ *
+ * @codeCoverageIgnore
  */
 class InvoiceFixtures extends Fixture
 {