Add check_fks argument to Wagtail's get_latest_revision_as_object method.

[josewails]

The model_from_serializable_data in django-model-cluster allows the check_fks argument, ensuring referenced foreign objects exist in the database. In Wagtail, get_latest_revision_as_object eventually invokes model_from_serializable_data, setting check_fks as True by default. With numerous tags, this results in extensive SQL queries for a single page. Would it make sense to add a check_fks argument in get_latest_revision_as_object, defaulting to True?