Add an initial store model

The attributes are all the ones that will be supported for the initial
preferences-onto-store refactoring

Related to #4415
Create a store on new/store migration

Grabbed the defaults from the existing preferences.

Related to #4415
Switch config editor to modify Spree::Store

Related to #4415
Switch examples off of site_name

With site_name now on store, we need to track something so it may as
well be track_inventory_level as an example

Related to #4415
Switch app config test to move off of site_name

Related to #4415
Switch Spree::Config to use new store attributes
don't override existing configuration methods

Related to #4415
meta_keywords delegated to description

Related to #4415