We have a number of pages in the administration UI today that make use

of subforms, which are forms dynamically hidden or shown based on

the selection on another form control. Each one of these had to be

written manually, which made implementations inconsistent and sometimes

buggy.
This change introduces a new formal subform implementation, which we'll

be able to start using in the settings pages. There are several new

things introduced here:


An -is-subform modifier on rb-c-form-fieldset, which turns a

   fieldset into a subform.


Data attributes for specifying a subform group, ID, and a controller

   element ID (which is used to automatically manage subform displays).


Guidance on setting both the disabled and hidden attributes to

   ensure the form's data won't be validated or sent to the server, and

   hiding the form from display and from screen readers (note that

hidden is an HTML 5.1 attribute, which is available in all our

   supported browsers, including IE11).


Support in RB.FormView for automatically setting up and managing

   subform state based on the data attributes.


Standard templates for representing a form instance as a subform,

   and a dictionary of subform IDs to forms as subforms.


Upcoming changes will update other forms to use this, fixing our form

validation issues on Django 1.11.

Unit tests pass.
Made use of this to update all the existing subforms (which will be

in an upcoming change). Verified everything worked as expected.