add meta tag for google verification

lms/templates/main.html

@@ -19,6 +19,7 @@
   <%static:css group='ie-fixes'/>
   <![endif]-->
   <meta name="path_prefix" content="${MITX_ROOT_URL}">
+  <meta name="google-site-verification" content="_mipQ4AtZQDNmbtOkwehQDOgCxUUV2fb_C0b6wbiRHY" />
 
   % if not course:
   <%include file="google_analytics.html" />