[OAuth2] Don't send unnecessary modhash to OAuth clients
OAuth2: Expose several endpoints

Endpoints getting "read" scope:
* /api/search_reddit_names
* /api/subreddits_by_topic
* /api/recommend/sr/{srnames}
* /subreddits
* /subreddits/new
* /subreddits/popular

Endpoint getting "modconfig" scope:
* /r/{subreddit}/about/edit
/dev/api: Put /api/store_visits in the API docs

api_doc notes' plumbing was changed slightly so
that VGold() endpoints automatically state that
they require a gold subscription in the docs.

"save" OAuth2 scope added to the store_visits endpoint.