Rewrite get_form_types() to not require fiddling with metaclasses to populate FORM_MODEL_CLASSES
Remove validate_usage from the wagtailforms.backends API. Since non-expert users are likely to be inheriting from AbstractEmailForm rather than hooking up backends to their own custom form models, it is of marginal usefulness and not worth the added complexity of the metaclass setup
no point in making form a property of the form page, since its scope is entirely within the 'serve' method
Replace wagtailforms.backends model with a simple inheritance model
merge branch 'form-builder' of github.com:torchbox/wagtail into form-builder