People on a waiting list should get an email when an activity is cancelled

[supertom01]

Currently enrolled people will receive an email when an activity is cancelled, including instructions with regards to refunds and etc. However, at this moment it is not yet done for people that are on the waiting list.

The email function used for this has already an argument for it, but the view controlling it does not set it yet. It should be quite easy to collect the people on the waiting list and then process them in a similar way.