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