[FIX] website_slides: fix website_meta_og_img right on partial channel

156955de 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.