FR: improved post approval fail message

[Catting]

at https://github.com/e621ng/e621ng/blob/master/app/controllers/moderator/post/approvals_controller.rb#L18
flash[:notice] = "You can't approve this post"
should only show up if the post is not already approved. If it is already approved, do something like:
flash[:notice] = "Post was already approved"