PLT-6139 (Server): Private Channel member managing (#5941)

Adds an EE policy feature to allow restricting system-wide which level of
Admins can manage the membership of private channels.

model/authorization.go

@@ -305,7 +305,6 @@ func InitalizeRoles() {
 		[]string{
 			PERMISSION_READ_CHANNEL.Id,
 			PERMISSION_MANAGE_PUBLIC_CHANNEL_MEMBERS.Id,
-			PERMISSION_MANAGE_PRIVATE_CHANNEL_MEMBERS.Id,
 			PERMISSION_UPLOAD_FILE.Id,
 			PERMISSION_GET_PUBLIC_LINK.Id,
 			PERMISSION_CREATE_POST.Id,