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/helpers/events_helper.rb

@@ -39,4 +39,12 @@ module EventsHelper
       content_tag :span, 'REPLACEMENT', class: (['label', 'label-info'] + label_classes)
     end
   end
+
+  def track_selector_input(form)
+    if @program.tracks.any?
+      form.input :track_id, as: :select,
+                            collection: @program.tracks.where(state: 'confirmed', cfp_active: true).pluck(:name, :id),
+                            include_blank: true
+    end
+  end
 end