Add rough form selector before showing post form

Add a screen before 'create post' to select form. Add addition view, and
some very rough handling for this in ModalController.
Re-add user fields to post create form

Add user fields to form schema.
Tweak styling for nested fieldsets
Fix form validation

Make sure form doesn't actually submit if we hit js errors.
Make sure we validate the model, not just schema.
Make nested forms get defined early to avoid JS errors when validating
with
fields that were never rendered
Add basic required/not-required validation for custom fields

Adds dynamically generated validation rules based on form.
Update backbone-validation to handle model.validation as a function vs
an object