Fixed countdown and projector update issues

* agenda_item.get_absolute_url('projector') returns the activate-url of the related slide.
* agenda_item.is_activate() returns True, if the related item is active
* Fixed set_active_slide to accept kwargs
* Reset countdown when saving a new duration time
* Update countdown overlay when begin_speak and end_speak is called
* Fixed blinking countdown

Fixes: #1078, #1076, #1075