iqdb: lower similarity cutoff, return more results (fix #4190).

* Change cutoffs on upload page to max 5 results, min. 20% similarity.
* Change cutoffs on standalone /iqdb_queries page to max 20 results, min. 0% similarity.
* /iqdb_queries.json: add `limit` and `similarity` params to change default cutoffs.

app/logical/iqdb_proxy.rb

@@ -1,12 +1,15 @@
 class IqdbProxy
-  def self.query(image_url)
+  def self.query(url, limit, similarity)
     raise NotImplementedError unless Danbooru.config.iqdbs_server.present?
 
-    url = URI.parse(Danbooru.config.iqdbs_server)
-    url.path = "/similar"
-    url.query = {url: image_url}.to_query
-    json = HTTParty.get(url.to_s, Danbooru.config.httparty_options).parsed_response
-    decorate_posts(json)
+    limit ||= 20
+    similarity ||= 0.0
+    query = { url: url, limit: limit }
+    response = HTTParty.get("#{Danbooru.config.iqdbs_server}/similar", query: query, **Danbooru.config.httparty_options)
+
+    json = decorate_posts(response.parsed_response)
+    json = json.select { |result| result["score"] >= similarity.to_f }.take(limit.to_i)
+    json
   end
 
   def self.decorate_posts(json)