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 ?
I added `preview: true` there but the setting still shows up for non-dimagi users. Is there another change I need to make to get that option to work?