CWRExport does not defer CWR loading

[matijakolaric]

There are a number of problems here, which must be all solved in order for defer to work properly:

• [x] CWRExport.created_on must become a proper DateTime field.
• [x] Migration must include filling out this information from CWR when possible
• [x] CWRExportAdmin.date must rely on this field
• [x] All instances of "if obj.cwr" must be changed to "if obj.year"
• [x] Similar problems with ACKImportAdmin