Refactor Reportbooru API clients.

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

app/views/static/sitemap.xml.erb

@@ -16,7 +16,7 @@
    </url>
 
    <% cache("sitemap", :expires_in => 24.hours) do %>
-      <% @popular_search_service.each_search do |tags, count| %>
+      <% @reportbooru_service.post_search_rankings(Date.yesterday) do |tags, count| %>
          <url>
             <loc><%= posts_url(tags: tags) %></loc>
             <lastmod><%= Date.today %></lastmod>
@@ -38,4 +38,4 @@
          </url>
       <% end %>
    <% end %>
-</urlset>
+</urlset>