Improve collapsible HTMLForm styling (and accessibility, slightly)

Improve collapsible HTMLForm styling (and accessibility, slightly)

Previously the form was only collapsible/expandable using the small
link-button on the far right. Now you can click on the entire header
to do it, and it has an appropriate pretty icon.

We add appropriate ARIA `role` and textual labels. Together with
recently added `toggleARIA` option of jquery.makeCollapsible in
Ic457bda58e56f we feature a screen reader workable output.

Bug: T222904
Change-Id: I6964296bc6870550478de662d21f12a1fc084c15