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/javascript/src/styles/specific/uploads.scss

@@ -9,10 +9,19 @@ div#c-uploads {
       margin-bottom: 2em;
     }
 
-    ul#links {
+    ul#upload-image-metadata, ul#links {
       margin-bottom: 1em;
     }
 
+    div#related-posts-by-source, div#iqdb-similar {
+      display: inline-block;
+
+      .hint {
+        white-space: nowrap;
+        max-width: 100%;
+      }
+    }
+
     label[for="upload_as_pending"] {
       display: inline;
     }