Add a Content Cards widget

[khromov]

As per:
https://wordpress.org/support/topic/content-cards-in-sidebar-widgets?replies=1#post-7614559

Shouldn't be too hard, just a standard width with a text input that runs the CC shortcode callback.

[ideag]

as with other places that are not posts, there is a question of where to store OpenGraph data. Of, course, add_post_meta() does not actually check if post with that particular $post_id exists, so I guess we could store such data with $post_id = 0 or something along these lines.