The broadcast user permissions are changing, so to avoid confusion and
permissions which exist in the database but don't display on the
frontend we are going to remove all existing permissions for users of
broadcast services. This also updates the permissions of invited users
who are still pending.
The exception to this is the `view_activity` permission, which we always
add for broadcast users even if they have no other permissions.
(aad017a184/app/main/forms.py (L1043))
