Branch v2 1242 force shib login

For institutions with force_shib_login flag enabled users that belongs to these institutions are not allowed to login or register locally - forcing them to use CAFe.

refs #1242
How about:

```ruby
def check_shib_login_only
  if params.has_key?(:user)
    institution_id = params[:user][:institution_id]
    institution = Institution.where(id: institution_id).first
    if institution && institution.force_shib_login?
      flash[:error] = I18n.t('users.registrations.shibboleth.error.force_shib_registration')
      redirect_to root_path
    end
  end
end
```

I'm not 100% sure, but only redirecting should be enough to abort the execution. It doesn't need to return `true` or `false`. See http://stackoverflow.com/questions/20623616/what-does-before-action-returning-false-do-in-rails-4