Add support for being able to stop content from showing in the search results after a certain period of time

There is a need to be able to have certain blog posts and calendar events to automatically stop showing up in search results on a website after a certain period of time. Sometimes old blog posts that aren't as relevant show up higher in search results than they should. One way to deal with this would be to have those items automatically expire from search results after a certain date or time period. This can apply to both the Search feature on BranchCMS websites as well as search engines like Google. For the internal search this could be done by setting an expiration date on the blog post when it's being indexed. For external search engines, this would involve automatically setting a `` tag to prevent indexing.