email notification for canceled bookings

[schrandy]

Hey,
users get emails for every action they do on cb, but for cancelling bookings. For consistency it would be good to have the option to send an email to the user after cancellation.

[flegfleg]

planned for 1.0

[sgrubsmyon]

Implemented this in https://github.com/sgrubsmyon/commons-booking/tree/email_on_cancel