Fix a copy-paste error in the "only-published" API authorization class.
Merge pull request #216 from birkenfeld/fix-api-authorization

Fix a copy-paste error in the "only-published" API authorization class.
added `is_published` to Pages admin list
staff users can see pages in draft. Close #169
404 if event location is not found.
404 if EventCategory is not found
Merge pull request #219 from fcurella/features/drafts

staff users can see pages in draft. Close #169
added convenience button to edit page for staff users. Close #191
refactored `iso_time_tag` from a filter to an inclusion tag
fixed test for 03b50a0a
added `Event.previous_time`
fixed missed event time in template
fixed times for past events in detail template
removed unexacting fields from events.Event