Add list of default subreddits

[kgellci]

I think Reddit has sunset their default subreddits list, but we can still pre-populate the side navigation with a defaults list. Can probably start by using this one: https://www.reddit.com/r/defaults/comments/61mgs9/list_of_default_subreddits_26_march_2017/

We can make the changes here for now: https://github.com/kgellci/Area51/blob/master/ListingService/Sources/Subreddit.swift

I plan on refactoring to make less static fo future use but fine to use for now.

[lucashoeft]

To keep the UX similar to the website, I would like to have two sections in the table view for now. First with "All", "Popular" and "News" and the second with the list you posted except for news as it is already in the first section.