AudienceEditController does not separate concerns very well

Our AudienceEditController is a large controller which does not separate concerns. In master, this controller is already over 400 lines of code and in our most recent dev commit, it is even over 600 lines of code.

To fix this, the following tasks will be performed:

Participant-related actions will be moved to a separate new controller (AudienceParticipantController?)
Document exporting and importing will be moved to a separate controller (again) (AudienceDocumentController)
AudienceEditController will be changed to only support all leftover functionality, such as editing the audience data itself.

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