Customizable token for anonymous audiences (4)

Currently, the token for anonymous audience cannot be changed. It would be nice to let the organizer choose the token. Related to issue #13 (closed).

• Change the "Rename" button of an anonymous audience to "Edit" (1).
• Add the possibility to change the token to the "Edit" (old "Rename) modal (1).
• Make sure the back-end accepts and saves the new token (1).
• Make sure that the accepted tokens are unique and limited to /[A-Za-z0-9\-]{3,255}/ (1)