Modify CFP to accept proposals for other things

Add field cfp_type and relevant validations
Add Program#cfp to preserve backwards compatibility
Add 'for_events' scope to the cfp, in order for it to be used like
program.cfps.events
Add useful methods
Add rspec test for the new code
The supported cfp types can be viewed via Cfp::TYPES

db/migrate/20170530072155_add_type_to_cfps.rb

@@ -0,0 +1,15 @@
+class AddTypeToCfps < ActiveRecord::Migration
+  class TmpCfp < ActiveRecord::Base
+    self.table_name = 'cfps'
+  end
+
+  def change
+    add_column :cfps, :cfp_type, :string
+
+    TmpCfp.reset_column_information
+    TmpCfp.find_each do |cfp|
+      cfp.cfp_type = 'events'
+      cfp.save!
+    end
+  end
+end