Add ProgrammingGroup & ProgrammingGroupMembership

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

db/migrate/20230710131250_create_programming_groups.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class CreateProgrammingGroups < ActiveRecord::Migration[7.0]
+  def change
+    create_table :programming_groups do |t|
+      t.belongs_to :exercise, foreign_key: true, null: false, index: true
+
+      t.timestamps
+    end
+  end
+end