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/fixtures/readability.json

@@ -10,13 +10,6 @@
             "article__title": "We Looked At 167,943 Tweets & Found Out Hashtags Are Worthless",
             "archive": false
         },
-        {
-            "article__title": "Réfugiés: l'UE va créer 100 000 places d'accueil dans les Balkans",
-            "article__url": "http://www.liberation.fr/planete/2015/10/26/refugies-l-ue-va-creer-100-000-places-d-accueil-dans-les-balkans_1408867",
-            "archive": false,
-            "date_added": "2016-09-08T11:55:58+0200",
-            "favorite": false
-        },
         {
             "article__title": "No title found",
             "article__url": "http://news.nationalgeographic.com/2016/02/160211-albatrosses-mothers-babies-animals-science/&sf20739758=1",