[FIX] website_slides: fix website_meta_og_img right on partial channel

156955de6b introduced a new property `website_meta_og_img` in
`website.seo.metadata`.

When rendering a template, `render()` is calling `get_website_meta` that will
end up reading `website.seo.metadata` fields.

But `website_slides` module is overriding `check_field_access_rights` to allow
partial read on the record.
The new `website_meta_og_img` was missing from the allowed fields, thus
crashing when an user without the required slide's group would try to read the
slide.

addons/website_slides/models/slides.py

@@ -270,7 +270,7 @@ class Slide(models.Model):
     _PROMOTIONAL_FIELDS = [
         '__last_update', 'name', 'image_thumb', 'image_medium', 'slide_type', 'total_views', 'category_id',
         'channel_id', 'description', 'tag_ids', 'write_date', 'create_date',
-        'website_published', 'website_url', 'website_meta_title', 'website_meta_description', 'website_meta_keywords']
+        'website_published', 'website_url', 'website_meta_title', 'website_meta_description', 'website_meta_keywords', 'website_meta_og_img']
 
     _sql_constraints = [
         ('name_uniq', 'UNIQUE(channel_id, name)', 'The slide name must be unique within a channel')