Create a more powerful export feature

It is necessary for some after-event analytics use-cases to export participant data. The current implementation can only export the contents of the participant table.

The proposed solution would allow organisers to open an 'Export Participants' dialog, where they could specify which fields from the participants' UserProfile and Registration models they want exported. The Export function should be able to export custom fields from a Registration as well.

The exported data should be easily parseable by spreadsheet software, eg.: by creating a well-formed .csv document.