Allow confirmed speakers to register anytime

If the registration period is over then speakers of confirmed events
can't register anymore. They should be able to, since they are gonna be
there anyway.

* Allow the speakers of confirmed events to register regardless of the
registration period
* Provision for registrations of confirmed speakers, so as not to
exceed the registration limit
* Add scope for registered speakers

Fix #829 and fix #802

app/models/program.rb

@@ -46,6 +46,10 @@ class Program < ActiveRecord::Base
     def confirmed
       joins(:events).where(events: { state: :confirmed })
     end
+
+    def registered(conference)
+      joins(:registrations).where('registrations.conference_id = ?', conference.id)
+    end
   end
 
   accepts_nested_attributes_for :event_types, allow_destroy: true