Add unregistered speakers scope to program

And use it in Conference#registration_limit_exceeded?

Also change self.registered(conference) to registered(conference) to fix failures in linters

app/models/conference.rb

@@ -719,7 +719,7 @@ class Conference < ApplicationRecord
   # * +True+ -> If the registration limit has been reached or exceeded
   # * +False+ -> If the registration limit hasn't been exceeded
   def registration_limit_exceeded?
-    registration_limit > 0 && registrations.count + program.speakers.confirmed.count - program.speakers.confirmed.registered(program.conference).count >= registration_limit
+    registration_limit > 0 && registrations.count + program.speakers.confirmed.unregistered(program.conference).count >= registration_limit
   end
 
   # Returns an hexadecimal color given a collection. The returned color changed

app/models/program.rb

@@ -51,6 +51,10 @@ class Program < ApplicationRecord
     def registered(conference)
       joins(:registrations).where('registrations.conference_id = ?', conference.id)
     end
+
+    def unregistered(conference)
+      self - registered(conference)
+    end
   end
 
   accepts_nested_attributes_for :event_types, allow_destroy: true