Adds `picture_url` method to `EssencePicture`.

Returns the url to show the picture and handles the crop sizes and secure hash automatically.
You only have to pass the size, if the image should be resized. Otherwise the full size will be used.
Refactors `render_essence_picture_view` so it uses the new `EssencePicture#picture_url`.
Adds `attachment_url` method to `EssenceFile`.

That returns the download attachment path.
Adds serializers for all essences and related models.
Only serialize published elements.
Adds a `named` scope to `Content`