Regression introduced in cca43aa4d1

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

Fixes #2579.

See #1674.

Pass communication event type code to OrderPlacementMixin.get_message_context()

The procedural views were deprecated in Django 1.11 and will be removed in Django 2.1.

Fixes #2547.

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.