Show old weight and new weight in weight system notes
After !21515 (merged), we're tracking weight change events in the database, rather than as text in system notes. This means that, for cases where we have an event for the current and previous weight, change the note to be something like:
changed weight from 2 to 3*
Or even:
removed weight (was 4)
!21515 (comment 265834413) and !21515 (comment 265401305) also talk about this.
We wouldn't change previous system notes, and when we only have the current weight event (because we didn't track any previous ones), we could still match the current style.