Rework (projector) template with bootstrap.

- Always use templatetag 'absolute_url' instead of (depricated) 'model_url'.
  Search and replace all old arguments: edit -> update, view -> detail, delete
- Fixed bug in setting default value in 'absolute_url' templatetag
  (default value is defined in models.py only).

Updated 'backend' template.
Merge pull request #901 from emanuelschuetze/newprojectortemplate

Rework projector template with bootstrap
Added projector.api.register_slide_model

Some fixes for the projector.
More projector fixtures

* Update projector when polls are updated.
* Update projector when assignment candidates are updated
* Update projector when motion submitter are updated
* Fixed the assignment publish poll view