events on instance overview [changes config]

This adds the possibility to add an events pager to the instance overview page.

In a first step the config options concerning the overview page are changed from booleans to lists. This makes them more compact and also allows us to set an ordering. This is done for both main content and sidebar (sidebar can currently only contain `u'events'` or nothing). **Please note that this is not backwards-compatible!**

In a second step the new option `u'events'` for main content is introduced.