Allow other fields to be send using API

Entry API can now have these new fields:
- content
- language
- preview_picture
- published_at

Re-use the ContentProxy to be able to do the same using the web UI (in the future).
htmLawed is used to clean stuff from content, I hope it’ll be enough to avoid security breach.

Lower content validation when we want to update an entry with content already defined. Before, language & content_type were required. If there weren’t provided, we re-fetched the content using graby. I think these fields aren’t required for an entry to be created. So I removed them.
Which means some import from the v1 export won’t be re-fetched since they provide content, url & title.

Also, remove liberation link from Readability import to avoid overlaping import (from wallabag v1, which had the same link)

tests/Wallabag/ImportBundle/Controller/WallabagV2ControllerTest.php

@@ -119,9 +119,9 @@ class WallabagV2ControllerTest extends WallabagCoreTestCase
                 $this->getLoggedInUserId()
             );
 
-        $this->assertNotEmpty($content->getMimetype(), 'Mimetype for http://www.liberation.fr is ok');
-        $this->assertNotEmpty($content->getPreviewPicture(), 'Preview picture for http://www.liberation.fr is ok');
-        $this->assertNotEmpty($content->getLanguage(), 'Language for http://www.liberation.fr is ok');
+        $this->assertEmpty($content->getMimetype(), 'Mimetype for http://www.liberation.fr is empty');
+        $this->assertEmpty($content->getPreviewPicture(), 'Preview picture for http://www.liberation.fr is empty');
+        $this->assertEmpty($content->getLanguage(), 'Language for http://www.liberation.fr is empty');
 
         $tags = $content->getTags();
         $this->assertContains('foot', $tags, 'It includes the "foot" tag');