Fix stock status result

Was previously using hard coded min/max values for months of stock so
the reports were broken.

This required refactoring StockState and moving it to corehq from casexml. To do that there is a schema to add the model and a data migration to move the data in this PR, and a schema migration to delete the old model in the related casexml PR.

Depends on https://github.com/dimagi/casexml/pull/192