We say in the Terms that we will email users if they change significantly, but we don't actually have a way to do that. We need to add a feature that allows site administrators to do that.
- only needs to be exposed to site admins, not coordinators, so putting it in the admin interface is fine
- should be documented in docs/admin.py
- must be a manual trigger, not an automatic one, because computers can't tell whether a change is "significant"
Depends on T147431.