Revert "remove jquery include from cdn"

This reverts commit 1210241160.

app/views/layouts/default.html.erb

@@ -28,6 +28,7 @@
   <meta name="disable-post-tooltips" content="<%= CurrentUser.disable_post_tooltips? %>">
   <%= content_tag(:meta, nil, name: "disable-mobile-gestures", content: CurrentUser.disable_mobile_gestures?) %>
   <%= auto_discovery_link_tag :atom, posts_path(:format => "atom", :tags => params[:tags]) %>
+  <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
   <%= javascript_pack_tag "application" %>
   <%= stylesheet_pack_tag "application" %>
   <% if CurrentUser.user.custom_style.present? %>