uploads: add new source:<url> dupe check (fix #3873)

* On the /uploads/new page, instead of just showing a "This post has
probably already been uploaded" message, show the actual thumbnails of
posts having the same source as what the user is trying to upload.

* Move the iqdb results section up top, beside the related posts section.

app/views/uploads/new.html.erb

@@ -12,6 +12,7 @@
       <% end %>
 
       <%= render "image" %>
+      <%= render "related_posts", source: @source %>
       <%= render "sources/info" %>
 
       <div id="client-errors" class="error-messages ui-state-error ui-corner-all" style="display:none"></div>
@@ -90,16 +91,6 @@
           </div>
         </div>
 
-        <% if Danbooru.config.iqdbs_server %>
-          <% if params[:url] %>
-            <div class="input" id="iqdb-similar">
-              <p><em>Loading similar...</em></p>
-            </div>
-          <% else %>
-            <div class="input" id="iqdb-similar" style="display: none;"></div>
-          <% end %>
-        <% end %>
-
         <div class="input">
           <div>
             <%= f.label :tag_string, "Tags" %>