extended commit (see note)

- Added ``Store`` and related models.

- Added ``Customer.email_preference`` field.

- Added ``load-host-data`` command.

- Added ``Change`` model.

- Added ``rattail.db.record_changes()`` function.

- Added database synchronization service (Windows only).

extensive commit (see note)

The following changes are included:

- Initial batch support added (with ``PrintLabels`` provider).

- GPC data type added.

- Changed internal name of file monitor.

- Added progress support for label printing.

- Label profiles moved from config to database model.

- Removed ``rattail.db.init_database()`` function.

- Moved some enum values from db extension to core (``rattail.enum`` module).

- Improved SIL support: moved ``rattail.sil`` to subpackage, added ``Writer
class etc.