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 ?
Argh, I think I accidentally was editing `commcare-hq/staticfiles/app_manager/json/commcare-profile-settings.yaml` instead of `commcare-hq/corehq/apps/app_manager/static/app_manager/json/commcare-profile-settings.yaml`. So just adding `preview: true` does the trick after all.