[SS-2018-015] Update jQuery version

The old jQuery version is critically out of date, and there are no
blockers to updating it (utilised sections of the library are fully
backwards compatible). This commit updates to the latest stable release.

templates/Page.ss

@@ -37,7 +37,7 @@ Change it, enhance it and most importantly enjoy it!
 </div>
 <% include Footer %>
 
-<% require javascript('//code.jquery.com/jquery-1.7.2.min.js') %>
+<% require javascript('//code.jquery.com/jquery-3.3.1.min.js') %>
 <% require themedJavascript('script') %>
 
 </body>