Fix whitelisting subreddits from the NSFW filter

Reported in [this reddit post.](https://www.reddit.com/r/Enhancement/comments/2jx861/is_it_possible_to_have_only_certain_subreddits/)

Previously `body:not(.allowOver18) .thing.over18 { display: none !important; }` would override `.thing.over18.allowOver18 { display: block !important; }`

I believe the second line `.thing.over18.allowOver18 { display: block !important; }` is now redundant (removing it didn't seem to make a difference in testing), but I'm not sure.