Add ProgrammingGroup & ProgrammingGroupMembership

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

app/policies/programming_group_policy.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class ProgrammingGroupPolicy < ApplicationPolicy
+  def new?
+    everyone
+  end
+
+  def create?
+    everyone
+  end
+end