Allow export of Event fields with participants

When exporting a list of event participants, it would be extremely useful to be able to include fields specific to the event in the export in addition to those specific to the participant. In particular, we need to be able to export the event date, so when scanning through a list of participants we can see which day they volunteered. For a political campaign, you have hundreds of events that might just be called "Door Knocking", and we've had to put the date in the name to get useful lists, but then it looks stupid on the calendar, so that's not ideal.

Just add "Event" fields to those that can be selected for export in addition to the "Participant" and "Individual" ones.