WL-3170 Meetings also assign sequence

Currently the sequence number is only assigned to timeslots. This is
is important for creating a cancellation type email, and so lets give a
sensible sequence number if the meeting is being cancelled.
WL-3170 Attendees won't cancel meetings

An ical file is sent to an organiser when a signup is cancelled by an
attendee to update the list of the attendees of the meeting. This should
not be a cancellation message because the meeting has not been cancelled.

The text of this email is quite confusing and should probably be fixed too.
WL-3170 Cancel? Pick the right event

When an attendee cancels their own signup the event is not picked up in
the code as it stands. Instead of looking for events that the user is
attending we should be using the internal list of removed events for the
class.

Of course the list will only have one element but how would the interface
know that now?