Remove `over18` keyword from user_subreddits and default_subreddits.
Remove `limit` keyword from default_subreddits.
Subreddit.add_props: only call default_subreddits when needed.

Also some cleanup.
Delete unused Subreddit.is_subscriber_defaults.
SubredditBar: Don't include popular subreddits if user is loggedin.
Add popular subreddits to top bar if user has few subscriptions.