Improve filtering of events to remove rejects or duplicates

The calendar view was showing some events I'd indicated I was going to attend.
New logic here filters those out better. I'm still being conservative and only
removing events I've explicitly declined just to be safe.

I also found a bit more about why recurring events were showing up twice and were
hard to annotate. Each recurring event came twice, once with a unique ID and once
with an ID for the series. This code filters out the series versions so we just
get annotatable one-off events.