Implement track request acceptance

Allow track submitter to request specific dates
Redirect to Tracks#edit if a track doesn't have a room or start/end date
before accepting it

Don't allow the submitter or the track organizers to edit the request
after it has been accepted or confirmed

Restrict track selection in proposals and move track selection from
Proposals form to events helper

Mark cfp_active of the tracks table as not null and fill in true if nil

app/models/event.rb

@@ -45,6 +45,7 @@ class Event < ActiveRecord::Base
   validates :max_attendees, numericality: { only_integer: true, greater_than_or_equal_to: 1, allow_nil: true }
 
   validate :max_attendees_no_more_than_room_size
+  validate :acceptable_track
 
   scope :confirmed, -> { where(state: 'confirmed') }
   scope :canceled, -> { where(state: 'canceled') }
@@ -303,4 +304,11 @@ class Event < ActiveRecord::Base
   def conference_id
     program.conference_id
   end
+
+  ##
+  # Allow only confirmed tracks that belong to the same program and are included in the cfp
+  def acceptable_track
+    return unless track && track.program && program
+    errors.add(:track, 'is invalid') unless track.confirmed? && track.cfp_active && track.program == program
+  end
 end