Added missing cms.page and site.sites from frozen Migration

With the latest changes to the inherit plugins migration, there are missing models in the frozen migration.  With a clean database or when trying to run tests, the migration fails with:

ValueError: Cannot successfully create field 'from_page' for model 'inheritpageplaceholder': "The model 'page' from the app 'cms' is not available in this migration.".

site.sites is also missing.

This pull request re-adds the necessary frozen models.