The Authentication 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 subforms.
As part of this, the base admin/settings.html template has been

set up to load in subforms. This will be necessary for upcoming changes

for other settings forms.

Tested saving the authentication form with each subform selection. Verified

that only the selected subforms received any client-side or server-side

validation, and that settings loaded.