Event pages should be set to NoIndex when event is not public or in the past

When an event is configured as not public, the event is excluded from public advertisement by iCalendar an the Upcoming Events block.

When enabled, this event will be included in iCalendar feeds and displayed on your site's "Upcoming Events" block.

Unfortunately the event (even very old and long closed events) are still indexed by search engines. Although we cannot force search engines to remove the listing of the event (actually unwanted pages can be removed by for instance Google Search Console).

This patch will inject a NOINDEX,NOFOLLOW in the head of the Event Info page when:

1. the event is set to not public
2. the event is in the past

https://github.com/civicrm/civicrm-core/pull/11496