Edit activities permission is unclear, needs improvement

1. there is a 'view all activities' and a 'delete all activities' permission, but there is no 'edit activities' permission

2. a contact cannot edit activities for which they are an assignee, target or creator if ANY OTHER contact besides their own is associated with the contact

3. use case: this makes it impossible for one contact to assign another contact an activity and have that contact be able to set it to 'completed'.

I argue that any activity when you can see and is assigned to you, you should be able to change the status.