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/logical/recommender_service.rb

@@ -0,0 +1,40 @@
+module RecommenderService
+  module_function
+
+  MIN_POST_FAVS = 5
+  MIN_USER_FAVS = 50
+  CACHE_LIFETIME = 4.hours
+
+  def enabled?
+    Danbooru.config.recommender_server.present?
+  end
+
+  def available_for_post?(post)
+    enabled? && post.fav_count > MIN_POST_FAVS
+  end
+
+  def available_for_user?(user)
+    enabled? && user.favorite_count > MIN_USER_FAVS
+  end
+
+  def recommend_for_user(user_id, limit = 50)
+    body, status = HttpartyCache.get("#{Danbooru.config.recommender_server}/recommend/#{user_id}", params: { limit: limit }, expiry: CACHE_LIFETIME)
+    return [] if status != 200
+
+    process_recs(body)
+  end
+
+  def recommend_for_post(post_id, limit = 50)
+    body, status = HttpartyCache.get("#{Danbooru.config.recommender_server}/similar/#{post_id}", params: { limit: limit }, expiry: CACHE_LIFETIME)
+    return [] if status != 200
+
+    process_recs(body).reject { |rec| rec[:post].id == post_id }
+  end
+
+  def process_recs(recs)
+    recs = JSON.parse(recs).to_h
+    recs = Post.where(id: recs.keys).map { |post| { score: recs[post.id], post: post } }
+    recs = recs.sort_by { |rec| -rec[:score] }
+    recs
+  end
+end