Bug: a smith member can change_owner_key before go_online

The bug reported at Issue #245 (closed) is not fully fixed by Merge Request !277 (merged).

The association IdtyIndex -> AccountId is created when a smith member sets their session keys: https://git.duniter.org/nodes/rust/duniter-v2s/-/blob/master/pallets/authority-members/src/lib.rs#L333.

Then, the smith member can change their AccountId by calling Identity.change_owner_key. The OwnerKeyChangePermission check will pass because the smith member is not online yet.

Historically, I created a public function, change_owner_key, in the authority-members pallet to update the IdtyIndex -> AccountId association: !86 (diffs).

However, for some reason, the OnIdtyChangeHandler was removed, which introduced a bug.