Unfortunately, a few of our showcase websites don't seem to exist
anymore. We should add a few more again.

Oscaro released an integration for the Adyen payment gateway. Let's show
the world!

Thank you, @metamatik.

The chocolate box have updated their URL (dropped 'the'). The readme
still refers to the old url for which the SSL certificate has expired.

Tangent Snowball currently doesn't have plans to work on those, and
they're in a very early stage. Hence it makes sense to just drop them
from the list.

Crate.io is no more, and testsupport has been deprecated for a long time
now.

Live Oscar site by, err, me.

We might have to give it visibility in the docs as well, but I did not
find an appropriate place.

The #django-oscar channel has been in operation for some time.

- Presentations link
- Commercial support update

- Add Freetix screenshot
- Copy reworking
- Drop link to Mirumee :(

(Should have tested some of this before pushing...)

Also bring logos into repo (rather than imgshack).

Building the sandbox was explained in multiple locations, and in
slightly different ways. The sandbox now has a separate document.

Fix #581