"Edited" timestamp is updated when transforming / resolving comments

This is a follow-up to https://gitlab.com/gitlab-org/gitlab-ce/issues/57330

When transforming / resolving an already edited note, the displayed edited timestamp is updated to the current time.

This is because we made last_edited_at as an alias of updated_at. updated_at needs to be touched for polling to work so we cannot change that behaviour.

What we can do is create a new edited_at / last_edited_at column and track the actual edit timestamps there.

This may also need a huge background migration to migrate the data.

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The override to edited? in app/models/note.rb should also be removed once this is done since that would be unneeded. This was used as a workaround to fix gitlab-ce#57330