WebStyle: serve 503 HTTP code when in maintenance
WebStyle: serve 503 HTTP code when in maintenance

* If the site is in the maintenance mode, i.e. if
  CFG_ACCESS_CONTROL_LEVEL_SITE > 1, then always serve a 503
  HTTP error code by default, so that crawlers don't index
  the site. (closes #805)

inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio