is_required and sequence_number are removed. That also makes the unused
get_prerequisites function obsolete.

Those fields were added with the idea that those event flows are linear,
but all implementations assume a non-linear pattern.

Fixes #745

- Add more links
- Add event details to line detail page

This allows updating code in both places, if necessary. Getting Django
1.5 support would be painful otherwise. This also removes the circular
dependency of django-oscar-testsupport on Oscar.

The requirements were merged. The imports were updated
accordingly. Unused imports in the touched files were removed. No
further changes.

As they save stuff...