rename movie_poster -> movie_poster_(medium)

BUR #49925 has been approved by @nonamethanks.

create alias movie_poster -> movie_poster_(medium)
remove alias movie_poster -> movie_poster_(medium)
deprecate movie_poster

Since we have movie_poster_(object), we should make it clearer that movie_poster is meant for the medium.

This bulk update request is pending automatic rejection in 5 days.

The bulk update request #49925 (forum #395637) has been approved by @nonamethanks.