System notes for changing the description are combined even if edits were made by different users

We combine system notes for description edits when multiple edits are made in a short period of time.

We should only combine system notes when the edit is made by the same user.

Also, we shouldn't update a system note that's not the latest because that may cause diffs to be out of order.

For example:

1. user_1 edits the description, system note 1 is added.
2. After 1 minute, user_2 edits the description, system note 2 is added because this is a different user editing
3. After another minute, user_1 edits the description again. We should create a new system note here and not edit system note 1 so that we won't have problems when we implement diffs in https://gitlab.com/gitlab-org/gitlab-ce/issues/65776