fixes some issues related to commtrack feature preview flag

http://manage.dimagi.com/default.asp?151233

easiest read commit by commit.

the main changes are:
- don't toggle the location preview when disabling commtrack: https://github.com/dimagi/commcare-hq/commit/b95fc126bde09d70746fffcebaacfbce2b14b5c6
- when copying a commtrack project from the exchange make sure to set the feature previews properly. https://github.com/dimagi/commcare-hq/commit/cfae263490a329470a2f36a2fe2fd27f99785067

the rest is basically cleanup.

depends on https://github.com/dimagi/toggle/pull/11
F401 'namespaced_item' imported but unused