Add events for pair programming study

app/models/event.rb

@@ -3,8 +3,19 @@
 class Event < ApplicationRecord
   include Creation
   belongs_to :exercise
-  belongs_to :file, class_name: 'CodeOcean::File'
+  belongs_to :file, class_name: 'CodeOcean::File', optional: true
+  belongs_to :study_group, optional: true
+  belongs_to :programming_group, optional: true
 
   validates :category, presence: true
-  validates :data, presence: true
+
+  # We temporary allow an event to be stored without data.
+  # This is useful if the category (together with the user and exercise) is already enough.
+  validates :data, presence: true, if: -> { %w[pp_start_chat pp_invalid_partners pp_work_alone].exclude?(category) }
+
+  before_validation :data_presence
+
+  def data_presence
+    self.data = data.presence
+  end
 end