Hide "Post Updated" message when post/page body is changed

[SecGen]

Sometimes when editing posts/pages, and saving along the way, due to the "Post Updated" or "Post Draft Updated" message staying visible even after the body of the post has been changed, it's possible for the user to think that the recently amended post had already been saved.

I'd suggest hiding the "Post Updated" item as soon as a change is made to the title or body. Then when looking at the top of the post, the user isn't misled to thinking that the changed post has already been saved/updated. This just makes for good UI.

[solarissmoke]

See point 2 on #17028.