Add tagged services for import

- list services in /import
- add url to import service
- ImportBundle routing are now prefixed by /import
- optimize flush in each import (flushing each 20 contents)
- improve design of each import
- add more tests

src/Wallabag/ImportBundle/Import/ImportInterface.php

@@ -13,6 +13,13 @@ interface ImportInterface extends LoggerAwareInterface
      */
     public function getName();
 
+    /**
+     * Url to start the import.
+     *
+     * @return string
+     */
+    public function getUrl();
+
     /**
      * Description of the import.
      *