Implement Adyen payment processor module

This squeezes a large number of commits over time, and should at this
point contain a more or less complete Adyen processor. There are some
things around reports that still need to be merged, once we have some
real reports to look at. Notifications are at this point not fully tested,
since we have no place to send them in the testing environment yet.

Cron jobs are not yet merged, that will send summary reports and such
things. Instead, I will verify it manually each day in the beginning,
and then write the scripts :)
Add placeholders for adyen notification user/pwd
Avoid error when showing date
Accept notifications without amounts
Fix one more date rendering in the admin interface
Include info about if a notification is live or test in the admin listing