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`.

src/Wallabag/CoreBundle/Helper/ContentProxy.php

@@ -76,6 +76,7 @@ class ContentProxy
         // Not sure what are the other possible cases where this property is empty
         if (empty($entry->getUrl()) && !empty($url)) {
             $entry->setUrl($url);
+            $entry->setGivenUrl($url);
         }
 
         $this->stockEntry($entry, $content);