Extract Contributor concern as abstract class

During documentation of the pair programming feature, we noticed that the Contributor should be an abstract class, which is parent for the User and the ProgrammingGroup. With this commit, we perform these changes.

app/models/programming_group.rb

@@ -1,19 +1,14 @@
 # frozen_string_literal: true
 
-class ProgrammingGroup < ApplicationRecord
-  include Contributor
-
-  has_many :anomaly_notifications, as: :contributor, dependent: :destroy
+class ProgrammingGroup < Contributor
   has_many :programming_group_memberships, dependent: :destroy
   has_many :external_users, through: :programming_group_memberships, source_type: 'ExternalUser', source: :user
   has_many :internal_users, through: :programming_group_memberships, source_type: 'InternalUser', source: :user
-  has_many :testruns, through: :submissions
-  has_many :runners, as: :contributor, dependent: :destroy
-  has_many :events, dependent: :destroy
+  has_many :testruns, through: :submissions # Only a single user starts testruns, but the group has access to them through their submissions.
+  has_many :events, dependent: :destroy # Only a single user creates events, but the group might be attributed to them optionally.
   has_many :events_synchronized_editor, class_name: 'Event::SynchronizedEditor', dependent: :destroy
   has_many :pair_programming_exercise_feedbacks, dependent: :destroy
   has_many :pair_programming_waiting_users, dependent: :destroy
-  has_many :user_exercise_interventions, as: :contributor
   belongs_to :exercise
 
   validate :min_group_size
@@ -21,14 +16,6 @@ class ProgrammingGroup < ApplicationRecord
   validate :no_erroneous_users
   accepts_nested_attributes_for :programming_group_memberships
 
-  def external_user?
-    false
-  end
-
-  def internal_user?
-    false
-  end
-
   def learner?
     true
   end
@@ -45,10 +32,6 @@ class ProgrammingGroup < ApplicationRecord
     Exercise
   end
 
-  def programming_group?
-    true
-  end
-
   def add(user)
     # Accessing the `users` method here will preload all users, which is otherwise done during validation.
     internal_users << user if user.internal_user? && users.exclude?(user)
@@ -56,23 +39,10 @@ class ProgrammingGroup < ApplicationRecord
     user
   end
 
-  def to_s
-    displayname
-  end
-
   def displayname
     "Programming Group #{id}"
   end
 
-  def to_page_context
-    {
-      id:,
-      type: self.class.name,
-      consumer: nil, # A programming group is not associated with a consumer.
-      displayname:,
-    }
-  end
-
   def programming_partner_ids
     users.map(&:id_with_type)
   end