Check if a post has previously been sent to social media, by checking
the value of a `field_sent_to_social_media` field.
This field is hidden on the node add/edit forms, and populated when a
post is sent to social media. Once this happens, it will not be sent to
social media again.
This change also populates the field for all existing posts, so that
they won't be re-sent to social media either.
