Cancancan

https://github.com/openSUSE/osem/issues/23

Some issues that I have not been able to solve yet are:

* Ability is not applied(?)
https://github.com/differentreality/osem/blob/cancancan/app/models/ability.rb#L103
Browser shows the new commercial page, test fails: 
https://github.com/differentreality/osem/blob/cancancan/spec/features/commercials_spec.rb#L22

Failures:
  1) Commercial organizer behaves like adds and updates a commercial of a conference
     Failure/Error: select('SlideShare', from: 'commercial_commercial_type')
     Capybara::ElementNotFound:
       Unable to find select box "commercial_commercial_type"
     Shared Example Group: "adds and updates a commercial" called from ./spec/features/commercials_spec.rb:65
     # ./spec/features/commercials_spec.rb:23:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:60:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:59:in `block (2 levels) in <top (required)>'

* I got failing tests which I cannot reproduce in browser with commented lines in:
https://github.com/differentreality/osem/blob/cancancan/spec/features/ability_spec.rb#L86

i would suggest to make actions with no body - as one liners

like

```ruby
def show; end
``` 

That way it is super explicit, that body is defined via callbacks