Attendance running total

[kristanslack]

It would be really help to have a running total when attendance is being marked. That way, when we know that there were N visitors, we could simply add that to the running total - rather than needing to save and then go back in.

That is, we don't do a headcount (kinda relying on Jethro to do that after marking people present) - but we are always aware of how many visitors/guests are there.