Event archiver

As per Helsbib issue [#173](https://libraryding.lighthouseapp.com/projects/73202/tickets/173-kan-man-gore-sa-gamle-events-bliver-slette-og-dermed-ikke-vis-i-sogninger), 
I have added an event archiving feature on behalf of Helsingør Municipal Libraries.

Basically, if enabled, it unpublishes past events the day after they ended.

This to avoid them showing up in search results, tag pages and other inconvenient places.

This is an opt-in feature, so it'll only do anything if actively enabled.