On call of /actors:
- if you have no actors, you'll be redirected to create_actor (soon:
  wizard)
- if you only have 1 (one) actor, you'll directly see this actor.
- if you have more than one (>1) actors you'll be redirected to the list.

Todo:
- add a possibility to add a second actor if you are on the single actor
  view.
Added a hidden possibility to add a second actor, if you are on the single
actor view. The option is pretty hidden and not really user friendly - but
the client wished it to be "invisible".
Once you are signed in, root ("/") now links to either your actors or the
admin panel.
Devise routes to root after sign in now (instead of logins#show).
Localized it.