Move (un)mark_as_nsfw Moderatable->Submission.

The behaviour of marking as NSFW is specific to Submission.
Comments (which also inherits from Moderatable) cannot be
marked as NSFW.
Don't raise exception if user is author of post.

It's possible to (un)mark as nsfw if you're either a mod
of the subreddit or author of the submission.
Handle new dataformat of subreddit list.