- allow creation of offers of all types
- allow creation of vouchers with multiple offers
- allow creation of voucher sets of all usages, and with multiple offers

Co-authored-by: Basil Dubyk <samitnuk.work@gmail.com>
Co-authored-by: Alexander Gaevsky <sasha@sasha0.ru>
Co-authored-by: Joseph Wayodi <jwayodi@gmail.com>

So that migrations are not required for changes to the
"OSCAR_IMAGE_FOLDER" setting, for projects that have not forked the
"catalogue" app.

Update the TestDashboardIndexForAnonUser test case

Also refactor catalogue option group and option tests.

- Move Oscar app configs into Django app config classes
- Rename Oscar/Django apps' app config module
- Remove "oscar.get_core_apps()" and its app overriding
- Remove "oscar.OSCAR_CORE_APPS"
- Port app forking code to work with Django app registry
- Port class loading code to use app configs to get app properties

Also fix the max_length on the url field. The underlying model field has
a max_length of 100, so it is invalid to have a larger value here.

DRY out dashboard page update/create views.

Refactor tests - perform tests of form logic directly rather than indirectly through view code.

Fixes #2559.

The priority isn't strictly a "restriction", but this seems like the most sensible form to add it to.

Better to use filenames which are commonly used in the python word then
custom ones.

Latter one is deprecated

A dict_values object was returned instead of a list. See #2223

This provides a minimal login template to access the dashboard. This is
especially useful when the shop doesn't provide accounts.

See #1474