Various wiki api cleanups

Most issues were brought up by bboe.  Mainly changes to make the api consistent with the rest of the site and makes clients like PRAW less hackish for these endpoints.  

- Page listing content should be a list inside of a key, not just a straight up list.  WILL BREAK EXISTING CLIENTS.
- Subreddit name is added to various endpoints for easy access from responses. 
- Current revision ID should be included in page json.  Currently no real way to retrieve it.
- Last commit removes c.wiki_id, code cleanup, seem to recall it being in this pull request for a good reason..however..it was 3 months ago.