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

app/models/cfp.rb

@@ -1,6 +1,10 @@
 # cannot delete program if there are events submitted
 
 class Cfp < ActiveRecord::Base
+  TYPES = %w(events).freeze
+
+  scope :for_events, (-> { find_by(cfp_type: 'events') })
+
   has_paper_trail ignore: [:updated_at], meta: { conference_id: :conference_id }
   belongs_to :program
 
@@ -8,6 +12,15 @@ class Cfp < ActiveRecord::Base
   validates :start_date, :end_date, presence: true
   validate :before_end_of_conference
   validate :start_after_end_date
+  validates :cfp_type,
+            presence: true,
+            inclusion: {
+              in: TYPES
+            },
+            uniqueness: {
+              scope: :program,
+              case_sensitive: false
+            }
 
   ##
   # Checks whether cfp date is updated
@@ -55,6 +68,16 @@ class Cfp < ActiveRecord::Base
     result > 0 ? result : 0
   end
 
+  ##
+  # Checks if the call for papers is currently open
+  #
+  # ====Returns
+  # * +false+ -> If the CFP is not set or today isn't in the CFP period.
+  # * +true+ -> If today is in the CFP period.
+  def open?
+    (start_date..end_date).cover?(Date.current)
+  end
+
   private
 
   def before_end_of_conference