Removed application mode and added shunting.

- Changed authentication to support shunting based on session and
  cookie values.
- Changed ability away from application mode based abilities and
  to auth provider based abilities.
- Added shunting page for selecting which auth provider to use.
- Removed tests regarding application mode. Still need to clean up tests.
- Changed catalog controller to use abilities for access restricted
  searching.
- Changed ApplicationController#current_user to return walk-in users.
  This is a bit messy now since one has to check for current_user
  and current_user.anonymous? when one wants to see if a user is logged
  in or not. The problems arise from BL using current_or_guest_user and
  CanCan using only current_user and then creating a new user as a guest
  user itself.
- Changed the logic in display of user util links to adhere to the changes
  in ApplicationController#current_user.
Removed "rember my choice" from walk-in PC login.
Working on using abilities.

- Removed some of the checks on current_user and current_or_guest_user
  from user util links. Now using abilities instead.
- Renamed provider :cas to :dtu_cas
- Added walk_in property to user.
Fixed tests plus some cleanup

- Fixed tests that were broken after switching to new application
  behavior where users don't have to login. This resulted in a narrower
  result set when searching among other things.
- Cleaned up ability and application controller a bit.
- Renamed User#anonymous? to user#authenticated? thus reversing logic
- Changed SessionsController#destroy so it doesn't redirect to login page
  anymore but instead redirects to session[:return_url] if set and root_path
  otherwise. Also updated related test to reflect this.
Merge remote-tracking branch 'origin/master' into remove_application_mode