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/contributor.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+class Contributor < ApplicationRecord
+  self.abstract_class = true
+
+  has_many :anomaly_notifications, as: :contributor, dependent: :destroy
+  has_many :user_exercise_interventions, as: :contributor
+  has_many :runners, as: :contributor, dependent: :destroy
+
+  has_many :submissions, as: :contributor
+
+  def learner?
+    raise NotImplementedError
+  end
+
+  def teacher?
+    raise NotImplementedError
+  end
+
+  def admin?
+    raise NotImplementedError
+  end
+
+  def internal_user?
+    is_a?(InternalUser)
+  end
+
+  def external_user?
+    is_a?(ExternalUser)
+  end
+
+  def programming_group?
+    is_a?(ProgrammingGroup)
+  end
+
+  def to_s
+    displayname
+  end
+
+  def to_page_context
+    {
+      id:,
+      type: self.class.name,
+      consumer: try(:consumer)&.name, # Only a user is associated with a consumer.
+      displayname:,
+    }
+  end
+end