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/ApiBundle/Controller/EntryRestControllerTest.php

@@ -342,6 +342,9 @@ class EntryRestControllerTest extends WallabagApiTestCase
             'url' => 'http://www.lemonde.fr/pixels/article/2015/03/28/plongee-dans-l-univers-d-ingress-le-jeu-de-google-aux-frontieres-du-reel_4601155_4408996.html',
             'tags' => 'google',
             'title' => 'New title for my article',
+            'content' => 'my content',
+            'language' => 'de_DE',
+            'published_at' => '2016-09-08T11:55:58+0200',
         ]);
 
         $this->assertEquals(200, $this->client->getResponse()->getStatusCode());
@@ -355,6 +358,9 @@ class EntryRestControllerTest extends WallabagApiTestCase
         $this->assertEquals('New title for my article', $content['title']);
         $this->assertEquals(1, $content['user_id']);
         $this->assertCount(2, $content['tags']);
+        $this->assertSame('my content', $content['content']);
+        $this->assertSame('de_DE', $content['language']);
+        $this->assertSame('2016-09-08T11:55:58+0200', $content['published_at']);
     }
 
     public function testPostSameEntry()