Add Form.NestForm template to allow different theming for nested forms used in Object editor

Adds Form.NestedForm and uses it in editors/Object. NestedForm extends Form but uses a different template which doesn't have a form tag. This generates valid HTML, but leaves the flexibility to tweak the nested form template if needed.
I'm not 100% sure this is correct. I don't know enough about bootstrap, or have anywhere setup to test. @fonji maybe you could confirm?