Fixes#350 and fixes#404
This change abandons validation via EditableFormField::validateField, as it bypassed too many core validation mechanisms (RequiredFields, etc).
In order to enforce consistency of editable field validation, display rules have been hard-disabled when a field is marked as required.
Since this removes functionality, I have incremented the major version number
HTML uploads are only relevant for SilverStripe 3.1,
since they're disallowed by default from 3.2 onwards in
the File.allowed_extensions configuration already.
API change: refactored Form() into getFormFields(), getFormActions(), getRequiredFields()
ENHANCEMENT: added updateForm* extension hooks to allow customization of userforms
BUGFIX: gave custom scripts their own ID to enable it to be blocked.
