Rework of the projector with websocket

* Set a static projector title
* absolute_urls for the activate links
* update the projector when a slide changes (in save())
* insert the absolute_url template filter
* Preview to slides
* renamed is_active to is_active_slide
* The SlideMixin has to come before the PersonMixin
* Update list of speakers
* Render Countdown via JS
* Reconnect projector after connection lost
* Overlays can allways be active and do not appear in the widget
* Rewrote the clock as overlay
Merge pull request #889 from ostcar/projector

Rework of the projector with websocket