Add a site setting to disable notifications on category changes

Continuing the discussion from Any way to prevent notifications for post merges, moves?:

I keep running into this problem:

  • I select a few old topics
  • I change their category to a more general one
  • The general category is set to Watching First Post by default
  • Every user in my forum gets a new notification for an old topic

Last time this happened I got a huge email spike (and a sizeable bill from Mailgun) :pensive:

I’ve since learned I can follow this manual process to avoid this, but I need to re-categorize so rarely that I tend to forget about this side effect.

It would be great if I could enable a site setting in the admin console to prevent users from ever being notified of topic category changes.