The Storage 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.
In order for this to work, the existing settings form had to be split up

into subforms. Ideally, we'd have pluggable backends we'd load settings

forms from, but this is at least a step toward that.

Tested saving the search form with each storage backend subform selection.

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

server-side validation, and that settings loaded and saved.