/moderator/dashboard: optimize sql queries

* Converts queries to use active record instead of raw sql. This ensures
  that user objects are loaded by rails in the join, so that we
  don't have to issue `User.find` calls to load users one-by-one.

* Use `.includes` to preload associations used in the view, to avoid
  additional N+1 query problems (primarily, calls to link_to_user
  also causing users to be loaded one-by-one).

app/views/moderator/dashboards/_activity_appeal.html.erb

@@ -10,13 +10,13 @@
     </tr>
   </thead>
   <tbody>
-    <% @dashboard.appeals.each do |appeal| %>
+    <% @dashboard.appeals.each do |post| %>
       <tr>
-        <td><%= link_to image_tag(appeal.post.preview_file_url), post_path(appeal.post) %></td>
-        <td><%= mod_link_to_user appeal.post.uploader, :negative %></td>
-        <td><%= post_flag_reasons(appeal.post) %></td>
-        <td><%= post_appeal_reasons(appeal.post) %></td>
-        <td><%= appeal.post.score %></td>
+        <td><%= link_to image_tag(post.preview_file_url), post_path(post) %></td>
+        <td><%= mod_link_to_user post.uploader, :negative %></td>
+        <td><%= post_flag_reasons(post) %></td>
+        <td><%= post_appeal_reasons(post) %></td>
+        <td><%= post.score %></td>
       </tr>
     <% end %>
   </tbody>