CaseListMixin refactor

Addresses this ticket: http://manage.dimagi.com/default.asp?115291

I have moved the logic for the `CaseListMixin` out of the `ExpandedMobileWorkerFilter` and into the `CaseListMixin` as reconmended on [this ticket](http://manage.dimagi.com/default.asp?115291#BugEvent.735657) and in https://github.com/dimagi/commcare-hq/pull/4113#issuecomment-52794731.

Have any feedback @esoergel ?



you could also create `corehq/apps/es/groups.py` for some of this.