Add auto-archiving posts to the CMS

[gawdn]

As a regular user, I would like to time sensitive posts to be automatically archived so I can have a news feed with the most pertinent information.

Tasks

• Consider adding an attribute to posts in the CMS with an end date
• Update the template to not render posts with the attribute

Acceptance criteria

• When I add a post with an end date and the date passes, the post is hidden from the primary news feed but can be found in the archive.