Add ProgrammingGroup & ProgrammingGroupMembership

* User can create programming group with other users for exercise
* Submission is shared in a group
* Also adjust specs

app/models/programming_group_membership.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class ProgrammingGroupMembership < ApplicationRecord
+  belongs_to :user, polymorphic: true
+  belongs_to :programming_group
+
+  validate :unique_membership_for_exercise
+  validates :user_id, uniqueness: {scope: %i[programming_group_id user_type]}
+
+  def unique_membership_for_exercise
+    if user.programming_groups.where(exercise: programming_group.exercise).any?
+      errors.add(:base, :already_exists, id_with_type: user.id_with_type)
+    end
+  end
+end