We made a change to remove all permissions from users and invited users
when the broadcast service settings form is submitted
(https://github.com/alphagov/notifications-api/pull/3284). However, when the
form is submitted, notifications-admin always adds the `view_activity`
permission even if no permission boxes are ticked, so we don't want to
remove that one permission
(256c840b46/app/main/forms.py (L1042))