Update the Search Settings page for modern subform support.
Review Request #11064 — Created July 3, 2020 and submitted
The Search Settings page has been updated to work with the subform
support introduced in Review Board 4.0. This gives us a maintainable way
of allowing authentication forms to be added to the page and configured
without triggering any browser-side validation issues for unselected
Tested saving the search form with each search backend subform selection.
Verified that only the selected subforms received any client-side or
server-side validation, and that settings loaded and saved.