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/user.rb

@@ -1,12 +1,11 @@
 # frozen_string_literal: true
 
-class User < ApplicationRecord
+class User < Contributor
   self.abstract_class = true
 
   attr_reader :current_study_group_id
 
   belongs_to :consumer
-  has_many :anomaly_notifications, as: :contributor, dependent: :destroy
   has_many :authentication_token, dependent: :destroy
   has_many :comments, as: :user
   has_many :study_group_memberships, as: :user
@@ -15,15 +14,12 @@ class User < ApplicationRecord
   has_many :programming_groups, through: :programming_group_memberships, as: :user
   has_many :exercises, as: :user
   has_many :file_types, as: :user
-  has_many :submissions, as: :contributor
   has_many :participations, through: :submissions, source: :exercise, as: :user
   has_many :user_proxy_exercise_exercises, as: :user
-  has_many :user_exercise_interventions, as: :contributor
   has_many :testruns, as: :user
   has_many :interventions, through: :user_exercise_interventions
   has_many :remote_evaluation_mappings, as: :user
   has_many :request_for_comments, as: :user
-  has_many :runners, as: :contributor
   has_many :events
   has_many :events_synchronized_editor, class_name: 'Event::SynchronizedEditor'
   has_many :pair_programming_exercise_feedbacks
@@ -31,8 +27,6 @@ class User < ApplicationRecord
   has_one :codeharbor_link, dependent: :destroy
   accepts_nested_attributes_for :user_proxy_exercise_exercises
 
-  scope :with_submissions, -> { where('id IN (SELECT user_id FROM submissions)') }
-
   scope :in_study_group_of, lambda {|user|
                               unless user.admin?
                                 joins(:study_group_memberships)
@@ -46,18 +40,6 @@ class User < ApplicationRecord
 
   validates :platform_admin, inclusion: [true, false]
 
-  def internal_user?
-    is_a?(InternalUser)
-  end
-
-  def external_user?
-    is_a?(ExternalUser)
-  end
-
-  def programming_group?
-    false
-  end
-
   def learner?
     return true if current_study_group_id.nil?
 
@@ -86,19 +68,6 @@ class User < ApplicationRecord
     study_group_memberships.where(study_group: current_study_group_id).limit(1)
   end
 
-  def to_s
-    displayname
-  end
-
-  def to_page_context
-    {
-      id:,
-      type: self.class.name,
-      consumer: consumer.name,
-      displayname:,
-    }
-  end
-
   def self.find_by_id_with_type(id_with_type)
     if id_with_type[0].casecmp('e').zero?
       ExternalUser.find(id_with_type[1..])