Add fuzzy search setting

Adds a fuzzy search setting and a new setting option called "disabled_default". If the app build version is less than the "since" option, then this default is used instead of the normal "default" option.

Addresses http://manage.dimagi.com/default.asp?134130

NOTE: Don't merge until 2.15 is ready ?
@NoahCarnahan Rather than blocking the merge, I think you can just add `preview: true` here and it won't show up except for @dimagi.com users