Fix invalid HTML introduced by Foundation

See foundation/foundation-sites issue 12167, which was fixed in
Foundation 6.7.1. However, since Foundation 6.7 requires Node 12 and the
foundation-rails gem hasn't been maintained for a year and a half, we're
not updating for now.

app/views/layouts/_common_head.html.erb

@@ -1,6 +1,7 @@
 <meta charset="utf-8" />
 <meta name="viewport" content="width=device-width, initial-scale=1.0" />
 <meta http-equiv="X-UA-Compatible" content="IE=edge" />
+<meta name="generator" content="Zurb Foundation for Sites 6" class="foundation-mq">
 <title><%= content_for?(:title) ? yield(:title) : default_title %></title>
 <% if rtl? %>
   <%= stylesheet_link_tag "application-rtl" %>