Export participants

[kwinkunks]

We could use a view of the participants for an event, with the following info:

• Username
• Full name (we don't always get this, see issue Get participant's full name #26 )
• Email
• Shirt-size (if collected)
• Dietary (if collected)
• Project (if any) - could be useful

One way to do this could be an Export button on the Participants tab, which is only active if the user is an admin perhaps?