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/_image.html.erb

@@ -5,7 +5,7 @@
     <%= image_tag(@source.image_url, :title => "Preview", :id => "image") %>
   <% end %>
 
-  <ul>
+  <ul id="upload-image-metadata">
     <% if @remote_size %>
       <li><strong>Size:</strong> <%= number_to_human_size(@remote_size) %></li>
     <% end %>