Add button to publish directly into Facebook from backoffice.

* Add button and modal for facebook post.
* Add python-social-auth
* Add basic api application implementation.
* Add migration for field observations + remove unused template.
* Add Facebook fan page id setting from environtment
* Add Facebook post form in book edit view.
* Facebook image post achiaved.
* Change flow to be able to publish as the fan page.
* More translations for field names
Merge branch 'share-facebook' into develop