CYB-251 "Likes" not being scored

 * previous implementation depended on the record being created already existing
 * gets the poster's canvas ID from the Attachment or MediaUrl model instead.
 * in the spec, the after_create will overwrite the specified gallery_id so an update
    is done on that attribute

[CYB-251 "Likes" not being scored](https://jira.ets.berkeley.edu/jira/browse/CYB-251)