Add button to Gravity page

This makes sure that the user doesn't accidentally cause the lists
to update because they were curious and clicked the Update Lists
link on the sidebar. Also, they can update the lists multiple
times, if they so desire.

gravity.php

@@ -16,7 +16,8 @@
     Success!
 </div>
 
-<pre id="output" style="width: 100%; height: 100%;"></pre>
+<button class="btn btn-lg btn-primary btn-block" id="gravityBtn">Update Lists</button>
+<pre id="output" style="width: 100%; height: 100%;" hidden="true"></pre>
 
 <?php
     require "footer.php";