recommendations: open user recommendations to all users.

* Open recommendations to all users (not just gold).
* Show recommendations on all posts (not just posts after 2017).
* Allow users to browse recommendations for other users.
* Increase number of recommended posts returned.
* Change endpoints to /recommended_posts?user_id=1234 and
  /recommended_posts?post_id=1234 and add json/xml support.

app/models/recommender_service.rb

@@ -1,62 +0,0 @@
-module RecommenderService
-  extend self
-
-  SCORE_THRESHOLD = 5
-
-  def enabled?
-    Danbooru.config.recommender_server.present?
-  end
-
-  def available_for_post?(post)
-    return true if Rails.env.development?
-
-    enabled? && post.created_at > Date.civil(2017, 1, 1) && post.fav_count >= SCORE_THRESHOLD
-  end
-
-  def available_for_user?
-    enabled? && CurrentUser.is_gold?
-  end
-
-  def recommend_for_user(user_id)
-    ids = Cache.get("rsu:#{user_id}", 1.hour) do
-      resp = HTTParty.get(
-        "#{Danbooru.config.recommender_server}/recommend/#{user_id}", 
-        Danbooru.config.httparty_options.merge(
-          basic_auth: {
-            username: "danbooru", 
-            password: Danbooru.config.recommender_key
-          }
-        )
-      )
-      JSON.parse(resp.body)
-    end
-    Post.find(ids.map(&:first))
-  end
-
-  def recommend_for_post(post_id)
-    ids = Cache.get("rss:#{post_id}", 1.hour) do
-      resp = HTTParty.get(
-        "#{Danbooru.config.recommender_server}/similar/#{post_id}", 
-        Danbooru.config.httparty_options.merge(
-          basic_auth: {
-            username: "danbooru", 
-            password: Danbooru.config.recommender_key
-          }
-        )
-      )
-      JSON.parse(resp.body)
-    end
-    if ids.is_a?(Hash) # error state
-      return []
-    end
-    Post.find(ids.reject {|x| x[0] == post_id}.map(&:first))
-  end
-
-  def recommend(post_id: nil, user_id: nil)
-    if post_id
-      recommend_for_post(post_id)
-    elsif user_id
-      recommend_for_user(user_id)
-    end
-  end
-end