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/ImportChain.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace Wallabag\ImportBundle\Import;
+
+class ImportChain
+{
+    private $imports;
+
+    public function __construct()
+    {
+        $this->imports = [];
+    }
+
+    /**
+     * Add an import to the chain.
+     *
+     * @param ImportInterface $import
+     * @param string          $alias
+     */
+    public function addImport(ImportInterface $import, $alias)
+    {
+        $this->imports[$alias] = $import;
+    }
+
+    /**
+     * Get all imports.
+     *
+     * @return array<ImportInterface>
+     */
+    public function getAll()
+    {
+        return $this->imports;
+    }
+}