Refactor Reportbooru API clients.

* Combine MissedSearchService, PostViewCountService, and
  PopularSearchService into single ReportbooruService class.
* Use Danbooru::Http for these services instead of HTTParty.

test/unit/reportbooru_service_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+class ReportbooruServiceTest < ActiveSupport::TestCase
+  def setup
+    @service = ReportbooruService.new(reportbooru_server: "http://localhost:1234")
+    @post = create(:post)
+    @date = "2000-01-01"
+  end
+
+  context "#popular_posts" do
+    should "return the list of popular posts on success" do
+      body = "[[#{@post.id},100.0]]"
+      @service.http.expects(:get).with("http://localhost:1234/post_views/rank?date=#{@date}").returns(HTTP::Response.new(status: 200, body: body, version: "1.1"))
+
+      posts = @service.popular_posts(@date)
+      assert_equal([@post], posts)
+    end
+
+    should "return nothing on failure" do
+      @service.http.expects(:get).with("http://localhost:1234/post_views/rank?date=#{@date}").returns(HTTP::Response.new(status: 500, body: "", version: "1.1"))
+
+      assert_equal([], @service.popular_posts(@date))
+    end
+  end
+end