FIX: Don’t use `user_generated` images as avatar images in Oneboxed Twitter content (PR #13712)

By default, Twitter will return the URL for the avatar image of the tweet poster as the og:image value.

However, if the user_generated attribute is true, we should not use this as the avatar URL as this will be an URL of an image in the tweet itself (e.g., an image belonging to a tweeted news story).