OSCAR_SLUG_FUNCTION: Support string notation to set slugifier
Importing things in settings is discouraged. But until now,
OSCAR_SLUG_FUNCTION could only be set as a callable, which requires
importing. It's now been updated to use the import_string function.
Ensure Oscar always displays timezone-aware datetimes
In a few cases, django.templatefilters.date was used to format a
datetime as string. This filters expects localtime, but was always
passed time in UTC.
Introduced a helper function that should be used instead of the template
filter.