Add delete token UI for info page

web/templates/result.html

@@ -29,11 +29,16 @@
 
   <div class="warn">
     <h2>Delete token</h2>
-    <p>Shown once. Keep it if you want to remove the file before it expires.</p>
+    <p>
+      Shown once. Keep it alongside the share link: pasting it under
+      <strong>Remove this file</strong> on that page deletes the file before it
+      expires.
+    </p>
     <p class="mono wrap">{{.DeleteToken}}</p>
     <form method="post" action="{{.Base}}api/d/{{.Meta.ID}}/delete">
       <input type="hidden" name="token" value="{{.DeleteToken}}">
-      <button type="submit" class="danger">Delete it now</button>
+      <button type="submit" class="danger"
+              data-confirm="Delete {{.Meta.Filename}}? This cannot be undone.">Delete it now</button>
     </form>
   </div>