Add ProgrammingGroup & ProgrammingGroupMembership

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

spec/factories/programming_group.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :programming_group do
+    exercise factory: :math
+
+    after(:build) do |programming_group|
+      # Do not change anything if users were provided explicitly
+      next if programming_group.users.present?
+
+      programming_group.users = build_list(:external_user, 2)
+    end
+  end
+end

spec/factories/submission.rb

@@ -10,6 +10,11 @@ FactoryBot.define do
       submission.exercise.files.editable.visible.each do |file|
         submission.add_file(content: file.content, file_id: file.id)
       end
+
+      # Do not change anything if a study group was provided explicitly or user has no study groups
+      next if submission.study_group.present? || submission.users.first.study_groups.blank?
+
+      submission.update!(study_group: submission.users.first.study_groups.first)
     end
   end
 end