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 ?
Not that I know of. Is the [is_previewer variable](https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/app_manager/templates/app_manager/app_view.html#L28-28) getting set to `false` for non-dimagi users?