Added given_url in entry table

- Added index on entry table for given_url field
- Fix tests:

    The previous `bit.ly` url redirected to doc.wallabag but that url doesn't exist in the fixtures.
    I used our own internal "redirector" to create a redirect to an url which exist in the fixtures.

Also, updating current migration to use the new `WallabagMigration`.

app/DoctrineMigrations/Version20170710125843.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace Application\Migrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Wallabag\CoreBundle\Doctrine\WallabagMigration;
+
+/**
+ * Added `given_url` field in entry table.
+ */
+class Version20170710125843 extends WallabagMigration
+{
+    /**
+     * @param Schema $schema
+     */
+    public function up(Schema $schema)
+    {
+        $entryTable = $schema->getTable($this->getTable('entry'));
+
+        $this->skipIf($entryTable->hasColumn('given_url'), 'It seems that you already played this migration.');
+
+        $entryTable->addColumn('given_url', 'text', [
+            'notnull' => false,
+        ]);
+    }
+
+    /**
+     * @param Schema $schema
+     */
+    public function down(Schema $schema)
+    {
+        $entryTable = $schema->getTable($this->getTable('entry'));
+
+        $this->skipIf(!$entryTable->hasColumn('given_url'), 'It seems that you already played this migration.');
+
+        $entryTable->dropColumn('given_url');
+    }
+}