Configure OSEM via environment variables

Instead of reading a yaml file we get configuration from the
environment.

app/models/ability.rb

@@ -45,7 +45,7 @@ class Ability
     # can view Commercials of confirmed Events
     can :show, Commercial, commercialable_type: 'Event', commercialable_id: Event.where(state: 'confirmed').pluck(:id)
     can [:show, :create], User
-    unless CONFIG['authentication']['ichain']['enabled']
+    unless ENV['OSEM_ICHAIN_ENABLED'] == 'true'
       can :show, Registration do |registration|
         registration.new_record?
       end