Add ProgrammingGroup & ProgrammingGroupMembership

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

db/migrate/20230710132428_create_programming_group_memberships.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class CreateProgrammingGroupMemberships < ActiveRecord::Migration[7.0]
+  def change
+    create_table :programming_group_memberships, id: :uuid do |t|
+      t.belongs_to :programming_group, foreign_key: true, null: false, index: true
+      t.belongs_to :user, polymorphic: true, null: false, index: true
+
+      t.timestamps
+    end
+  end
+end